Method and device for transmitting image information, and decoding method and device using same

ABSTRACT

An image decoding method according to the present invention comprises the steps of: acquiring information to form a reference picture set of a current picture by entropy decoding the received bitstream information; and performing prediction on a prediction block inside the current picture by using a reference picture list which is formed based on the reference picture set.

TECHNICAL FIELD

The present invention relates to video processing, and moreparticularly, to method and device for efficiently transmitting videoinformation and decoding method and device using the same.

BACKGROUND ART

Recently, demands for a high-resolution and high-quality video haveincreased in various fields of applications. As a video has higherresolution and higher quality, an amount of information on the videoincreases more.

Accordingly, when video information is transferred using media such asexisting wired or wireless broadband lines or video information isstored in existing storage media, the information transfer cost and theinformation storage cost increase.

High-efficiency video compressing techniques can be used to effectivelytransfer, store, and reproduce information on high-resolution andhigh-quality videos.

Inter prediction and intra prediction can be used to enhance videocompression efficiency. In the inter prediction, pixel values of acurrent picture are predicted with reference to information of otherpictures. In the intra prediction, pixel values of a current picture arepredicted using inter-pixel relationships in the same picture.

When the inter prediction is carried out, a video encoder and a videodecoder can perform a prediction process on the basis of a referencepicture list specifying reference pictures which can be used for acurrent block (current picture).

Information for constructing a reference picture list can be transmittedfrom the video encoder to the video decoder. The video decoder canconstruct a reference picture list on the basis of the informationreceived from the video encoder and can effectively perform the interprediction.

SUMMARY OF THE INVENTION Technical Problem

An object of the present invention is to provide video encoding methodand device which can improve video encoding/decoding efficiency.

Another object of the present invention is to provide video decodingmethod and device which can improve video encoding/decoding efficiency.

Still another object of the present invention is to provide videoinformation transmitting method and device which can improve videoencoding/decoding efficiency.

Still another object of the present invention is to provide predictionmethod and device which can improve video encoding/decoding efficiency.

Still another object of the present invention is to provide referencepicture list constructing method and device which can improve videoencoding/decoding efficiency.

Solution to Problem

According to an aspect of the present invention, there is provided avideo decoding method. The video decoding method may include the stepsof: acquiring information for constructing a reference picture set of acurrent picture by entropy-decoding received bitstream information; andperforming a prediction operation on a prediction target block in thecurrent picture using a reference picture list constructed on the basisof the reference picture set. Here, the information for constructing thereference picture set may include first flag information specifying along-term reference picture (LTRP) is used to perform an interprediction operation on at least one picture belonging to a currentsequence, and the first flag information may be transmitted through asequence parameter set (SPS) sequence by sequence.

When the first flag information specifies that a long-term referencepicture (LTRP) is used to perform an inter prediction operation on atleast one picture belonging to the current sequence, the referencepicture set may include a long-term reference picture (LTRP) set, andthe information for constructing the reference picture set may furtherinclude information for deriving a picture order count (POC) of an LTRPto be included in the LTRP set.

The information for deriving the POC may include a sequence parameterset (SPS) LTRP index information specifying an LTRP to be included inthe LTRP set out of one or more SPS candidate LTRPs to be transmittedthrough an SPS, the POC of the LTRP to be included in the LTRP set maybe derived on the basis of a least significant bit (LSB) of the POC ofthe LTRP specified by the SPS LTRP index information, and the SPS LTRPindex information may be transmitted through a slice headercorresponding to the current picture.

The information for constructing the reference picture set may includesecond flag information specifying whether the LTRP specified by the SPSLTRP index information is used as a reference picture of a picturebelonging to the current sequence, the LTRP set may not include the LTRPspecified by the SPS LTRP index information when the second flaginformation specifies that the LTRP specified by the SPS LTRP indexinformation is not used as a reference picture of the picture belongingto the current sequence, and the second flag information may betransmitted through a SPS sequence by sequence.

The SPS LTRP index information may be encoded and transmitted on thebasis of a fixed number of bits.

The information for deriving the POC may include PCO LSB informationspecifying the LSB of the POC of the LTRP to be included in the LTRPset, the POC of the LTRP to be included in the LTRP set may be derivedon the basis of the POC LSB information, and the POC LSB information maybe transmitted through a slice header corresponding to the currentpicture.

The information for constructing the reference picture set may includethird flag information specifying whether the LTRP having the POCderived on the basis of the POC LSB information is used as a referencepicture of the current picture, the LTRP set may not include the LTRPhaving the POC derived on the basis of the POC LSB information when thethird flag information specifies that the LTRP having the POC derived onthe basis of the POC LSB information is not used as a reference pictureof the current picture, and the third flag information may betransmitted through a slice header corresponding to the current picture.

According to another aspect of the present invention, there is provideda video decoder. The video decoder may include: an entropy decodingmodule that acquires information for constructing a reference pictureset of a current picture by entropy-decoding received bitstreaminformation; a prediction module that constructing a predicted blockcorresponding to a prediction target block in the current picture byperforming a prediction operation on the prediction target block using areference picture list constructed on the basis of the reference pictureset; and a reconstructed block constructing unit that constructs areconstructed block on the basis of the predicted block. Here, theinformation for constructing the reference picture set may include firstflag information specifying whether a long-term reference picture (LTRP)is used to perform an inter prediction operation on at least one picturebelonging to a current sequence, and the first flag information may betransmitted through a sequence parameter set (SPS) sequence by sequence.

According to still another aspect of the present invention, there isprovided a video information transmitting method. The video informationtransmitting method may include the steps of: performing an interprediction operation on a current picture using a reference picturelist; and entropy-encoding and transmitting information for constructinga reference picture set used to construct the reference picture list.Here, the information for constructing the reference picture set mayinclude first flag information specifying whether a long-term referencepicture (LTRP) is used to perform an inter prediction operation on atleast one picture belonging to a current sequence, and the step ofentropy-encoding and transmitting the information may includetransmitting the first flag information through a sequence parameter set(SPS) sequence by sequence.

When the first flag information specifies that a long-term referencepicture (LTRP) is used to perform an inter prediction operation on atleast one picture belonging to the current sequence, the referencepicture set may include a long-term reference picture (LTRP) set, andthe information for constructing the reference picture set may furtherinclude information for deriving a picture order count (POC) of an LTRPto be included in the LTRP set.

The information for deriving the POC may include a sequence parameterset (SPS) LTRP index information specifying an LTRP to be included inthe LTRP set out of one or more SPS candidate LTRPs to be transmittedthrough an SPS, and the step of entropy-encoding and transmitting theinformation may include transmitting SPS POC least significant bit (LSB)information specifying an LSB of the POC of each of the one or more SPScandidate LTRPs through the SPS sequence by sequence and transmittingthe SPS LTRP index information through a slice header corresponding tothe current picture.

The information for constructing the reference picture set may includesecond flag information specifying whether the LTRP specified by the SPSLTRP index information is used as a reference picture of a picturebelonging to the current sequence, the LTRP set may not include the LTRPspecified by the SPS LTRP index information when the second flaginformation specifies that the LTRP specified by the SPS LTRP indexinformation is not used as a reference picture of the picture belongingto the current sequence, and the step of entropy-encoding andtransmitting the information may include transmitting the second flaginformation through a SPS sequence by sequence.

The step of entropy-encoding and transmitting the information mayinclude entropy-encoding the SPS LTRP index information on the basis ofa fixed number of bits.

The information for deriving the POC may include PCO LSB informationspecifying the LSB of the POC of the LTRP to be included in the LTRPset, and the step of entropy-encoding and transmitting the informationmay include transmitting the POC LSB information through a slice headercorresponding to the current picture.

The information for constructing the reference picture set may includethird flag information specifying whether the LTRP having the POCcorresponding to the POC LSB information is used as a reference pictureof the current picture, the LTRP set may not include the LTRP having thePOC derived on the basis of the POC LSB information when the third flaginformation specifies that the LTRP having the POC corresponding to thePOC LSB information is not used as a reference picture of the currentpicture, and the step of entropy-encoding and transmitting theinformation may include transmitting the third flag information througha slice header corresponding to the current picture.

According to still another aspect of the present invention, there isprovided a video information transmitting device. The video informationtransmitting device may include: a prediction module that performs aninter prediction operation on a current picture using a referencepicture list; and an entropy encoding module that entropy-encodes andtransmits information for constructing a reference picture set used toconstruct the reference picture list. Here, the information forconstructing the reference picture set may include first flaginformation specifying whether a long-term reference picture (LTRP) isused to perform an inter prediction operation on at least one picturebelonging to a current sequence, and the entropy encoding module maytransmit the first flag information through a sequence parameter set(SPS) sequence by sequence.

Advantageous Effects

By employing the video encoding method according to the presentinvention, it is possible to improve the video encoding/decodingefficiency.

By employing the video decoding method according to the presentinvention, it is possible to improve the video encoding/decodingefficiency.

By employing the video information transmitting method according to thepresent invention, it is possible to improve the video encoding/decodingefficiency.

By employing the prediction method according to the present invention,it is possible to improve the video encoding/decoding efficiency.

By employing the reference picture list constructing method according tothe present invention, it is possible to improve the videoencoding/decoding efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating a video encoderaccording to an embodiment of the present invention.

FIG. 2 is a block diagram schematically illustrating a video decoderaccording to an embodiment of the present invention.

FIG. 3 is a diagram schematically illustrating examples of a candidateblock which can be used to perform an inter prediction process on acurrent block.

FIG. 4 is a flowchart schematically illustrating an example of a methodof constructing a reference picture list on the basis of a referencepicture set.

FIG. 5 is a block diagram schematically illustrating an example of adevice for initializing a reference picture list.

FIG. 6 is a flowchart schematically illustrating an encoding procedurewhich is performed by a video encoder according to the presentinvention.

FIG. 7 is a flowchart schematically illustrating a decoding procedurewhich is performed by a video decoder according to the presentinvention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention may be variously modified in various forms and may havevarious embodiments, and specific embodiments thereof will beillustrated in the drawings and described in detail. However, theseembodiments are not intended for limiting the invention. Terms used inthe below description are used to merely describe specific embodiments,but are not intended for limiting the technical spirit of the invention.An expression of a singular number includes an expression of a pluralnumber, so long as it is clearly read differently. Terms such as“include” and “have” in this description are intended for specifyingthat features, numbers, steps, operations, elements, components, orcombinations thereof used in the below description exist, and it shouldbe thus understood that the possibility of existence or addition of oneor more different features, numbers, steps, operations, elements,components, or combinations thereof is not excluded.

On the other hand, elements of the drawings described in the inventionare independently drawn for the purpose of convenience of explanation ondifferent specific functions in an image encoder and an image decoder,and do not mean that the elements are embodied by independent hardwareor independent software. For example, two or more elements out of theelements may be combined to form a single element, or one element may besplit into plural elements Embodiments in which the elements arecombined and/or split belong to the scope of the invention withoutdeparting from the concept of the invention.

Hereinafter, exemplary embodiments of the invention will be described indetail with reference to the accompanying drawings. The same elements inthe drawings will be referenced by the same reference signs and thedescription of the same elements will not be repeated.

FIG. 1 is a block diagram schematically illustrating a video encoderaccording to an embodiment of the present invention. Referring to FIG.1, a video encoder 100 includes a picture splitting module 105, aprediction module 110, a transform module 115, a quantization module120, a rearrangement module 125, an entropy encoding module 130, adequantization module 135, an inverse transform module 140, a filtermodule 145, and a memory 150.

The picture splitting module 105 may split an input picture into atleast one process unit. Here, the process unit may be a prediction unit(hereinafter, referred to as a “PU”), a transform unit (hereinafter,referred to as a “TU”), or a coding unit (hereinafter, referred to as a“CU”).

The prediction module 110 includes an inter prediction module thatperforms an inter prediction operation and an intra prediction modulethat performs an intra prediction operation. The prediction module 110may perform a prediction operation on the process unit of the picturesplit by the picture splitting module 105 to construct a predictedblock. The process unit of a picture in the prediction module 110 may bea CU, a TU, or a PU. It may be determined whether the predictionoperation performed on the corresponding process unit is an interprediction operation or an intra prediction operation, and specificdetails (for example, a prediction mode) of the prediction methods maybe determined. The process unit subjected to the prediction operationmay be different from the process unit of which the prediction methodand the specific details are determined. For example, the predictionmethod and the prediction mode may be determined in the units of PU andthe prediction operation may be performed in the units of TU.

In the inter prediction, a prediction operation may be performed on thebasis of information on at least one of a previous picture and/or asubsequent picture of a current picture to construct a predicted block.In the intra prediction, a prediction operation may be performed on thebasis of pixel information of a current picture to construct a predictedblock.

A skip mode, a merge mode, a motion vector prediction (MVP), or the likemay be used as the intra prediction method. In the inter prediction, areference picture may be selected for a PU, and a reference block havingthe same size as the PU may be selected. The reference block may beselected in the unit of integer pixels. A predicted block may beconstructed so that a residual signal from a current PU is minimized andthe magnitude of a motion vector is minimized.

The predicted block may be constructed in the unit of integer pixelsamples or in the unit of pixel samples less than an integer pixel, suchas ½ pixel samples and ¼ pixel samples. Here, a motion vector may alsobe expressed in the unit of pixel samples less than an integer pixel.For example, luma pixels may be expressed in the unit of ¼ pixels andchroma pixels may be expressed in the unit of ⅛ pixels.

Information such as an index of a reference picture selected through theinter prediction, a motion vector (for example, a motion vectorpredictor), and a residual signal may be entropy-encoded and transmittedto a video decoder. When the skip mode is used, the predicted block maybe used as a reconstructed block and thus the residual signal may not becreated, converted, quantized, and transmitted.

When the intra prediction is performed, a prediction mode may bedetermined in the unit of PUs and the prediction operation may beperformed in the unit of PUs. Alternatively, a prediction mode may bedetermined in the unit of PUs and the intra prediction may be performedin the unit of TUs.

The prediction modes in the intra prediction may include 33 directionalprediction modes and at least two non-directional modes. Thenon-directional modes may include a DC prediction mode and a planarmode.

In the intra prediction, a predicted block may be constructed after afilter is applied to reference samples. At this time, it may bedetermined whether a filter should be applied to reference samples,depending on the intra prediction mode of a current block and/or thesize of the current block.

A PU may be a block having various sizes/shapes. For example, in case ofthe inter prediction, a PU may be a 2N×2N block, a 2N×N block, a N×2Nblock, or a N×N block (where N is an integer). In case of the intraprediction, a PU may be a 2N×2N block or a N×N block (where N is aninteger). The PU having a block size of N×N may be set to be used inonly a specific case. For example, the PU having a block size of N×N maybe set to be used for only a CU having the smallest size or may be setto be used for only the intra prediction. In addition to theabove-mentioned sizes, PUs such as a N×mN block, a mN×N block, a 2N×mNblock, and a mN×2N block (where m<1) may be additionally defined andused.

Residual values (a residual block or a residual signal) between theconstructed predicted block and the original block may be input to thetransform module 115. The prediction mode information, the motion vectorinformation, and the like used for the prediction may be encoded alongwith the residual values by the entropy encoding module 130 and may betransmitted to the video decoder.

The transform module 115 may perform a transform operation on theresidual block by transform units and created transform coefficients.The transform unit in the transform module 115 may be a TU and may havea quad tree structure. The size of the transform unit may be determinedwithin a predetermined range of largest and smallest sizes. Thetransform module 115 may transform the residual block using a discretecosine transform (DCT) and/or a discrete sine transform (DST).

The quantization module 120 may quantize the residual values transformedby the transform module 115 and may create quantization coefficients.The values calculated by the quantization module 120 may be supplied tothe dequantization module 135 and the rearrangement module 125.

The rearrangement module 125 may rearrange the quantization coefficientssupplied from the quantization module 120. By rearranging thequantization coefficients, it is possible to enhance the codingefficiency in the entropy encoding module 130. The rearrangement module125 may rearrange the quantization coefficients in the form of atwo-dimensional block to the form of a one-dimensional vector through acoefficient scanning method. The rearrangement module 125 may enhancethe entropy encoding efficiency in the entropy encoding module 130 bychanging the coefficient scanning order on the basis of stochasticstatistics of the coefficients transmitted from the quantization module.

The entropy encoding module 130 may perform an entropy encodingoperation on the quantization coefficients rearranged by therearrangement module 125. Examples of the entropy encoding methodinclude an exponential golomb method, a CAVLC (Context-Adaptive VariableLength Coding) method, and a CABAC (Context-Adaptive Binary ArithmeticCoding) method. The entropy encoding module 130 may encode a variety ofinformation such as quantization coefficient information and block typeinformation of a CU, prediction mode information, split unitinformation, PU information, transfer unit information, motion vectorinformation, reference picture information, block interpolationinformation, and filtering information transmitted from therearrangement module 125 and the prediction module 110.

The entropy encoding module 130 may give a predetermined change to aparameter set or syntaxes to be transmitted, if necessary.

The dequantization module 135 dequantizes the values quantized by thequantization module 120. The inverse transform module 140 inverselytransforms the values dequantized by the dequantization module 135. Theresidual values created by the dequantization module 135 and the inversetransform module 140 may be merged with the predicted block predicted bythe prediction module 110 to construct a reconstructed block.

FIG. 1 illustrates that the residual block and the predicted block areadded by an adder to construct a reconstructed block. Here, the addermay be considered as a particular module (reconstructed blockconstructing module) that constructs a reconstructed block.

The filter module 145 may apply a deblocking filter, a sample adaptiveoffset (SAO), and/or an adaptive loop filter (ALF) to the reconstructedpicture.

The deblocking filter may remove a block distortion generated at theboundary between blocks in the reconstructed picture. The SAO mayreconstruct an offset difference of the residual block, which has beensubjected to the deblocking filter, from the original video in the unitof pixels and may be applied in the form of a band offset and an edgeoffset. The ALF may perform a filtering operation on the basis of theresultant values of comparison of the original picture with thereconstructed picture of which the blocks have been filtered by thedeblocking filter and/or the SAO. The ALF may be applied only when highefficiency is necessary.

On the other hand, the filter module 145 may not perform a filteringoperation on the reconstructed block used in the inter prediction.

The memory 150 may store the reconstructed block or picture calculatedby the filter module 145. The reconstructed block or picture stored inthe memory 150 may be supplied to the prediction module 110 thatperforms the inter prediction.

FIG. 2 is a block diagram schematically illustrating a video decoderaccording to an embodiment of the present invention. Referring to FIG.2, a video decoder 200 may include an entropy decoding module 210, arearrangement module 215, a dequantization module 220, an inversetransform module 225, a prediction module 230, a filter module 235, anda memory 240.

When a video bitstream is input from the video encoder, the inputbitstream may be decoded on the basis of the order in which videoinformation is processed by the video encoder.

For example, when the video encoder uses a variable length coding(hereinafter, referred to as “VLC”) method such as the CAVLC method toperform the entropy encoding operation, the entropy decoding module 210may implement the same VLC table as the VLC table used in the videoencoder and may perform the entropy decoding operation. When the videoencoder uses the CABAC method to perform the entropy encoding process,the entropy decoding module 210 may perform the entropy decodingoperation using the CABAC method to correspond thereto.

Information for constructing a predicted block out of the informationdecoded by the entropy decoding module 210 may be supplied to theprediction module 230, and the residual values entropy-decoded by theentropy decoding module 210 may be input to the rearrangement module215.

The rearrangement module 215 may rearrange the bitstream entropy-decodedby the entropy decoding module 210 on the basis of the rearrangementmethod in the video encoder. The rearrangement module 215 mayreconstruct and rearrange coefficients expressed in the form of aone-dimensional vector into coefficients in the form of atwo-dimensional block. The rearrangement module 215 may be supplied withinformation associated with the coefficient scanning performed by thevideo encoder and may perform the rearrangement using a method ofinversely scanning the coefficients on the basis of the scanning orderin which the scanning is performed by the video encoder.

The dequantization module 220 may perform dequantization on the basis ofthe quantization parameters supplied from the video encoder and thecoefficient values of the rearranged block.

The inverse transform module 225 may perform the inverse DCT and/orinverse DST of the DCT and/or DST, which has been performed by thetransform module of the video encoder, on the quantization result fromthe video encoder. The inverse transform may be performed on the basisof a transfer unit or a split unit of a picture determined by the videoencoder. The transform module of the video encoder may selectivelyperform the DCT and/or DST depending on plural information pieces suchas the prediction method, the size of a current block, and theprediction direction, and the inverse transform module 225 of the videodecoder may perform the inverse transform on the basis of the transforminformation on the transform performed by the transform module of thevideo encoder.

The prediction module 230 may construct a predicted block on the basisof prediction block construction information supplied from the entropydecoding module 210 and the previously-decoded block and/or pictureinformation supplied from the memory 240.

When the prediction mode of a current PU is an intra prediction mode,the prediction module 230 may perform an intra prediction operation ofconstructing a predicted block on the basis of pixel information of acurrent picture.

When the prediction mode for a current PU is the inter prediction mode,the prediction module 230 may perform the inter prediction operation onthe current PU on the basis of information included in at least one of aprevious picture and a subsequent picture of the current picture. Atthis time, motion information for the inter prediction of the currentPU, for example, information on motion vectors and reference pictureindices, supplied from the video encoder may be derived from a skipflag, a merge flag, and the like received from the video encoder.

The reconstructed block may be constructed using the predicted blockconstructed by the prediction module 230 and the residual block suppliedfrom the inverse transform module 225. FIG. 2 illustrates that theresidual block and the predicted block are added by an adder toconstruct a reconstructed block. Here, the adder may be considered as aparticular module (reconstructed block constructing module) thatconstructs a reconstructed block.

When the skip mode is used, the residual signal may not be transmittedand the predicted block may be used as a reconstructed block.

The reconstructed block and/or picture may be supplied to the filtermodule 235. The filter module 235 may perform a deblocking filteringoperation, an SAO operation, and/or an ALF operation on thereconstructed block and/or picture.

The memory 240 may store the reconstructed picture or block for use as areference picture or a reference block and may supply the reconstructedpicture to an output module.

Encoded or decoded pictures may be stored in a memory such as a decodedpicture buffer (DPB). When a current picture is encoded or decoded, theprevious pictures stored in the DPB may be referred to for performing aprediction operation on the current picture.

Specifically, the video encoder and the video decoder may stored thepreviously-encoded/decoded pictures in a reference picture list for usein the inter prediction.

When the inter prediction is performed, the video encoder and the videodecoder may perform a prediction operation on a target block (currentblock) of a current picture with reference to another picture. The interprediction may be performed by the prediction modes of the video encoderand the video decoder as illustrated in FIGS. 1 and 2.

When the inter prediction is performed, as described above, theprediction operation may be performed on the current block usinginformation of available neighboring blocks adjacent to the currentblock. Here, the neighboring blocks may include a “Col block” belongingto reference pictures which the current block can refer to. Here, theCol block may be determined on the basis of the relative position to the“co-located block” spatially located at the same position as the currentblock in a reference picture which the current block can refer to. Forexample, a storage unit covering pixels at predetermined relativepositions corresponding to the co-located block may be specified and theCol block may be a block covering the leftmost-top pixel in the storageunit.

In the below description, for the purpose of convenience of explanation,a neighboring block used to perform a prediction operation on a currentblock in the inter prediction is referred to as a “candidate block”.

In the inter prediction, the prediction operation may be performed onthe current block on the basis of information of candidates blocks. Incase of the skip mode or the merge mode, motion information (forexample, motion vector) and/or a reference picture for a candidate blockselected from the candidate blocks may be used as the motion informationand/or the reference picture for the current block.

When the MVP is performed, the motion information (for example, motionvector) for a selected candidate block may be used as a predicted valueof a motion vector for the current block, and the reference pictureinformation for the current block may be transmitted from the videoencoder to the video decoder. A motion vector difference (MVD) betweenthe MVP derived from the candidate block and the motion vector for thecurrent block may be transmitted from the video encoder to the videodecoder, and the prediction module of the video decoder may indue themotion information for the current block on the basis of the MVP and theMVD.

FIG. 3 is a diagram schematically illustrating an example of candidateblocks which can be used when the inter prediction is performed on acurrent block.

The prediction modules of the video encoder and the video decoder mayuse a neighboring block located at a predetermined position around acurrent block 400 as a candidate block. For example, in the example ofFIG. 3, two blocks A₀ 410 and A₁ 420 located on the left-bottom of thecurrent block and three blocks B_(o) 430, B₁ 440, and B₂ 450 located onthe right-top and the left-top of the current block may be selected asspatial candidate blocks. In addition to the blocks spatially adjacent,the Col block 460 may be used as a temporal candidate block.

On the other hand, regarding reference pictures used for the interprediction, reference pictures for the current blocks may be derivedfrom reference pictures of neighboring blocks or may be specified byinformation received from the video encoder. In case of the skip mode orthe merge mode, the prediction module of the video decoder may use thereference pictures of the neighboring blocks as the reference picturesfor the current block. When the MVP is applied, the prediction module ofthe video decoder may receive the information specifying the referencepictures for the current block from the video encoder.

Pictures encoded/decoded previously to the current picture may be storedin a memory (for example, a decoded picture buffer (DPB) and may be usedfor prediction of the current block (current picture). Pictures whichare available for the inter prediction of the current block may bestored in a reference picture list. Here, a reference picture to be usedfor the inter prediction of the current block among the referencepictures included in the reference picture list may be specified by areference picture index. That is, a reference picture index may mean anindex specifying a reference picture to be used for the inter predictionof the current block out of the reference pictures of the referencepicture list.

An I slice is a slice which is decoded through the intra prediction. A Pslice is a slice which is decoded through the intra prediction or theinter prediction using at most one motion vector and one referencepicture. A B slice is a slice which is decoded through the intraprediction or the inter prediction using at most two motion vectors andtwo reference pictures. Here, reference pictures may include ashort-term reference picture (hereinafter, referred to as STRP) and along-term reference picture (hereinafter, referred to as LTRP).

A short-term reference picture and a long-term reference picture may bereconstructed pictures stored in the DPB. A short-term reference picturemay be marked as “used for short-term reference” or “used forreference”. A long-term reference picture may be marked as “used forlong-term reference” or “used for reference”. For example, a pictureorder count difference between a picture to be decoded and a long-termreference picture may have a value in a range of “1” to “2²⁴−1”. Here,the picture order count (POC) may specify a picture display order.

Reference picture list 0 (hereinafter, referred to as “L0” for thepurpose of convenience of explanation) is a reference picture list usedfor the inter prediction of a P slice or a B slice. Reference picturelist 1 (hereinafter, referred to as “L1” for the purpose of convenienceof explanation) may be used for the inter prediction of a B slice.Therefore, uni-directional prediction based on L0 may be performed atthe time of inter prediction of a block of a P slice, and bi-directionalprediction (bi-prediction) based on L0 and L1 may be performed at thetime of inter prediction of a block of a B slice.

The video encoder and/or the video decoder may construct a referencepicture list when an encoding/decoding operation is performed on a Pslice and a B slice through the use of inter prediction. At this time, areference picture to be used for the inter prediction may be specifiedby a reference picture index. As described above, a reference pictureindex may mean an index specifying a reference picture to be used forthe inter prediction.

The reference picture list may be constructed on the basis of areference picture set determined or constructed by the video encoder andthe video decoder. The reference pictures constituting the referencepicture list may be stored in a memory (for example, DPB). The pictures(pictures encoded/decoded previously to a current picture) stored in thememory may be managed by the video encoder and the video decoder.

When a sliding window method is used as the method of managing thereference pictures, the reference pictures may be managed by a simplemethod of releasing a reference picture in a predetermined time passesafter the reference picture is stored in the memory, but this method hasseveral problems. For example, since there is a reference picture whichis not available any more, the reference picture may not be releasedfrom the memory and thus efficiency may be lowered. Since a storedreference picture is released from the memory after a predeterminedtime, it may be difficult to manage LTRPs.

A memory management command operation (MMCO) method of signalinginstruction information on management of reference pictures from thevideo encoder may be used in consideration of the problems of thesliding window method. Particularly, in the MMCO method, a command forallocating a picture to an LTRP, a command for changing the state of anLTRP to an STRP, and a command for marking an LTRP as “unused forreference” may be defined in regard to management of LTRPs.

However, when the MMCO method is used, picture loss may occur in thecourse of signaling. When a lost picture includes an MMCO command, thelost MMCO information may not be reconstructed and the memory (DPB) maynot be maintained in a state where currently-necessary pictures arecorrectly managed. Therefore, there is a possibility that the interprediction will be performed incorrectly.

In order to solve the above-mentioned problems, a method of transmittinginformation associated with a reference picture set necessary atdecoding a slice (and/or a picture) through a sequence parameter set(SPS), a picture parameter set (PPS), and/or a slice header may be used.At this time, the reference picture list may be constructed on the basisof the reference picture set.

A reference picture set may include reference pictures to be used forreference of a current picture/slice or a future picture/slice. Thereference pictures used at decoding a slice (and/or a picture) mayinclude an STRP and a LTRP. The STRP may include a forward short-termreference picture having a POC smaller than that of the current pictureand a backward short-term reference picture having a POC larger thanthat of the current picture. Here, the reference picture sets may bedetermined or constructed for each of the forward short-term referencepicture, the backward short-term reference picture, and the long-termreference picture.

In this specification, for the purpose of convenience of explanation, areference picture set for the forward short-term reference picture isreferred to as a forward short-term reference picture set, a referencepicture set for the backward short-term reference picture is referred toas a backward short-term reference picture set, and a reference pictureset for the long-term reference picture is referred to as a long-termreference picture set. For example, the forward short-term referencepicture set may be expressed by RefPicSetStCurrBefore, the backwardshort-term reference picture set may be expressed byRefPicSetStCurrAfter, and the long-term reference picture set may beexpressed by RefPicSetLtCurr.

FIG. 4 is a flowchart schematically illustrating an example of a methodof constructing a reference picture list on the basis of a referencepicture set.

In the example illustrated in FIG. 4, the operation of the video decoderis mainly described for the purpose of convenience of explanation. Areference picture list constructing step to be described later may beconsidered as a reference picture list initializing step.

Referring to FIG. 4, the video decoder may construct a reference pictureset on the basis of reference picture set information transmitted fromthe video encoder (S410). For example, the reference picture set may beconstructed for each picture to be subjected to inter prediction.

Here, the reference picture set may include the forward short-termreference picture set, the backward short-term reference picture set,and the long-term reference picture set. The reference pictures includedin each reference picture set may be specified by the POC. The POCindicates a picture display order.

The POCs of the reference pictures included in the forward short-termreference picture set and the backward short-term reference picture setmay be determined by relative POCs. Here, information on the relativePOC may be transmitted from the video encoder to the video decoder.

The relative POC may represent a POC difference between two pictures ina reference picture set. The relative POCs of the reference pictures(reference pictures having POCs smaller than that of the currentpicture) previous to the current picture in the POC order may correspondto POC differences from an immediately-previous reference picture in thereference picture set. The relative POCs of the reference pictures(reference pictures having POCs larger than that of the current picture)subsequent to the current picture in the POC order may correspond to POCdifferences from an immediately-previous reference picture in thereference picture set.

In the forward short-term reference picture set, forward short-termreference pictures having POC values smaller than the POC of the currentpicture may be located in the descending order of POCs. In other words,the pictures having POC values smaller than the POC of the currentpicture out of the pictures stored in the DPB may be located in thedescending order of POCs from the first position of the forwardshort-term reference picture set.

In the backward short-term reference picture set, backward short-termreference pictures having POC values larger than the POC of the currentpicture may be located in the ascending order of POCs. In other words,the pictures having POC values larger than the POC of the currentpicture out of the pictures stored in the DPB may be located in theascending order of POCs from the first position of the forwardshort-term reference picture set.

The reference pictures included in the long-term picture set may bedetermined on the basis of long-term reference picture set informationtransmitted from the video encoder. Here, the long-term referencepicture set information may include information for determiningreference pictures (and/or the POCs of the reference pictures) includedin the long-term reference picture set. Examples of the long-termreference picture set information transmitted from the video encoder tothe video decoder will be described later.

Referring to FIG. 4 again, the video decoder may construct a referencepicture list on the basis of the reference picture set (S420).

When reference picture list L0 is constructed, the video decoder mayconstruct the reference picture list by allocating reference pictureindices in the order of forward short-term reference picturesconstituting the forward short-term reference picture set, backwardshort-term reference pictures constituting the backward short-termreference picture set, and long-term reference pictures constituting thelong-term reference picture set. That is, forward short-term referencepictures may be allocated to reference picture list L0, then backwardshort-term reference pictures may be added thereto, and long-termreference pictures may be added thereto finally.

The forward short-term reference pictures constituting the forwardshort-term reference picture set may be added to reference picture listL0 in the same order as the order in which the forward short-termreference pictures are included in the forward short-term referencepicture set. That is, the forward short-term reference pictures may belocated in the descending order of POCs in reference picture list L0 andthe smaller the POC is, the larger value the reference picture index mayhave. The backward short-term reference pictures may be added toreference picture list L0 in the same order as the order in which thebackward short-term reference pictures are included in the backwardshort-term reference picture set. That is, the backward short-termreference pictures may be located in the ascending order of POCs inreference picture list L0 and the larger the POC is, the larger valuethe reference picture index may have. The long-term reference picturesmay be added to reference picture list L0 in the same order as the orderin which the long-term reference pictures are included in the long-termreference picture set.

In case of the B slice, reference picture list L1 may also beconstructed as well as reference picture list L0. When reference picturelist L1 is constructed, the video decoder may construct the referencepicture list by allocating the reference picture indices in the order ofthe backward short-term reference pictures constituting the backwardshort-term reference picture set, the forward short-term referencepictures constituting the forward short-term reference picture set, andthe long-term reference pictures constituting the long-term referencepicture set. That is, the backward short-term reference pictures may befirst allocated to reference picture list L1, then the forwardshort-term reference pictures may be added thereto, and the long-termreference pictures may finally be added thereto.

The backward short-term reference pictures constituting the backwardshort-term reference picture set may be added to reference picture listL1 in the same order as the order in which the backward short-termreference pictures are included in the backward short-term referencepicture set. That is, the backward short-term reference pictures may belocated in the ascending order of POCs in reference picture list L1 andthe larger the POC is, the larger value the reference picture index mayhave. The forward short-term reference pictures may be added toreference picture list L1 in the same order as the order in which theforward short-term reference pictures are included in the forwardshort-term reference picture set. That is, the forward short-termreference pictures may be located in the descending order of POCs inreference picture list L1 and the smaller the POC is, the larger valuethe reference picture index may have. The long-term reference picturesmay be added to reference picture list L1 in the same order as the orderin which the long-term reference pictures are included in the long-termreference picture set.

Reference picture indices may be sequentially allocated to the referencepictures added to reference picture lists L0 and L1.

The video decoder may use N reference pictures (reference pictureshaving reference picture indices 0 to N−1, where N is a natural number)from the first reference picture in the reference picture list asavailable reference pictures. Here, information on the number ofavailable reference pictures N may be transmitted from the videoencoder.

In the above-mentioned step, the reference picture list may beconsidered to being implicitly derived. When a reference picture list isimplicitly derived, the video encoder and the video decoder may derivethe reference picture list to be used for the inter prediction of thecurrent picture on the basis of the POCs of pictures as described above.

On the other hand, the video decoder may modify the implicitly-derivedreference picture list on the basis of information explicitlytransmitted from the video encoder. At this time, the video encoder maytransmit entry information specifying specific entries of the referencepicture list along with reference picture list modification informationspecifying that the implicitly-derived reference picture list ismodified. When the reference picture list is finally specified bymodifying the reference picture list on the basis of the informationexplicitly transmitted from the video encoder, it may be considered thatthe reference picture list is explicitly specified.

The video encoder may transmit entry information of L0 when L0 isspecified explicitly. The entry information of L0 may indicate referencepictures corresponding to indices in L0. The video encoder may transmitentry information of L1 when L1 is specified explicitly. The entryinformation of L1 may indicate reference pictures corresponding toindices in L1.

For example, when a reference picture list is explicitly specified bythe entry information, the order (and/or the reference picture indices)of forward short-term reference pictures, backward short-term referencepictures, and long-term reference pictures in the reference picture listmay be different from the order (and/or the reference picture indices)in the implicitly-derived reference picture list.

When the reference picture list is specified by the entry information,available reference pictures to be used may be different from those inthe implicitly-derived reference picture list.

When the reference picture list is explicitly specified, the videodecoder may construct the same reference picture list as the referencepicture list constructed by the video encoder on the basis of referencepicture list correction information and the entry information.

In the above-mentioned method of implicitly deriving a reference picturelist, the reference picture set and the reference picture list aredescribed in consideration of only available pictures for the purpose ofconvenience of explanation, but the video encoder and the video decodermay construct the reference picture set and/or the reference picturelist in consideration of availability of reference pictures.

FIG. 5 is a block diagram schematically illustrating an example of adevice for initializing a reference picture list.

In the example illustrated in FIG. 5, a reference picture listinitializing device 500 includes a reference picture set constructingunit 510 and a reference picture list constructing unit 520.

Referring to FIG. 5, the reference picture set constructing unit 510 mayconstruct a reference picture set-relevant on the basis of inputreference picture set information. For example, a reference picture setmay be constructed for every picture subjected to the inter prediction.Examples of the reference picture set-relevant information transmittedfrom the video encoder to the video decoder will be described later.

Here, the reference picture set may include a forward short-termreference picture set, a backward short-term reference picture set, andlong-term reference picture set. The reference pictures included in eachreference picture set may be specified by picture order count (POC). ThePOC indicates a picture display order.

Specific details of the operation which is performed by the referencepicture set constructing unit 510 is the same as described in the step(S410) of constructing a reference picture set in FIG. 4 and thus willnot be repeatedly described.

Referring to FIG. 5 again, the reference picture list constructing unit520 may construct a reference picture list on the basis of the referencepicture set. The reference picture list constructed by the referencepicture list constructing unit 520 may include L0 and L1. Specificdetails of the operation which is performed by the reference picturelist constructing unit 520 is the same as described in the step (S420)of constructing a reference picture list in FIG. 4 and thus will not berepeatedly described.

The reference picture list constructed by the reference picture listconstructing unit 520 may be stored in the DPB for prediction or may betransmitted to the prediction module for reference for prediction.

Here, it is described that the initialization of a reference picturelist is performed by another element (reference picture listinitializing device), but this is for the purpose of convenience ofexplanation and easy understanding of the present invention and thepresent invention is not limited to this configuration. For example, theinitialization of a reference picture list may be performed by thememories, for example, the DPBs, of the video encoder and the videodecoder described with reference to FIGS. 1 and 2. In this case, thereference picture list initializing device may be the DPBs. Theinitialization of a reference picture list may be performed by theprediction modules of the video encoder and the video decoder describedwith reference to FIGS. 1 and 2. In this case, the reference picturelist initializing device may be the prediction module. In addition, thereference picture list initializing device may be included as aparticular module in the video encoder and the video decoder.

In this specification, examples of a method of transmitting or signalinginformation on a long-term reference picture set of long-term referencepictures and constructing a long-term reference picture set will bedescribed below. A reference picture in the following description maymean a long-term reference picture and a long-term reference picture maybe referred to as an LTRP. In this specification, the signaling ofinformation may have a meaning of transmitting other information whichcan be used to derive the information as well as a meaning of explicitlyor directly transmitting information.

In this specification, the long-term reference picture set relevantinformation transmitted from the video encoder to the video decoder maybe described as plural syntax elements. Names of syntax elements andparameters defined in a sequence parameter set (SPS), a pictureparameter set (PPS), and/or a slice header are arbitrary and the syntaxelements and parameters having the functions identical or similarthereto are included in the scope of the present invention regardless ofthe names thereof.

For example, the video encoder may transmit LTRP_flag information in theunit of sequences specifying whether an LTRP is present in a currentbitstreamand/or a current sequence to the video decoder. A syntaxelement specifying the flag information may be defined in an SPS and maybe expressed as in Table 1.

TABLE 1 seq_parameter_set_rbsp( ) { Descriptor  ... use_long_term_reference_flag u(1)  ... }

In Table 1, use_long_term_reference_flag may be a flag specifyingwhether an LTRP is present in a current bit stream and/or a currentsequence. Referring to Table 1, use_long_term_reference_flag may bedefined as an SPS. For example, when use_long_term_reference_flag equal0, the flag may specify that an LTRP is not present in the current bitstream and/or the current sequence. When use_long_term_reference_flagequal 1, the flag may specify that an LTRP is present in the current bitstream and/or the current sequence. In this case, the video encoder maytransmit LTRP_flag information specifying whether an LTRP is present inthe current bit stream and/or the current sequence for each sequence.

When use_long_term reference_flag specifies that an LTRP is not presentin the current sequence, the video encoder may not transmit long-termreference picture set-relevant information other thanuse_long_term_reference_flag for the current sequence to the videodecoder. Therefore, the video decoder may not receive or process thelong-term reference picture set-relevant information other thanuse_long_term_reference_flag in the current sequence. Whenuse_long_term_reference_flag specifies that an LTRP is present in thecurrent sequence, the video encoder may additionally transmit long-termreference picture set-relevant information other thanuse_long_term_reference_flag for the current sequence to the videodecoder.

When the LTRP_flag information is transmitted in the unit of sequences,it may be determined in the unit of sequences whether an LTRP ispresent. That is, the value set as use_long_term_reference_flag may notbe changed in a sequence. Accordingly, when it is determined whether anLTRP is present in a current sequence, this determination result may bemaintained in the current sequence without being changed picture bypicture. In this case, since the video encoder transmits only one pieceof LTRP_flag information for one sequence, it is possible to reduce theamount of information transmitted from the video encoder to the videodecoder. Since the video decoder may not check whether an LTRP ispresent for each picture, it is possible to reduce complexity.

On the other hand, as described above, the name of the syntax elementdefined in the SPS as in the example shown in Table 1 is arbitrary andthe LTRP_flag information may be expressed by a syntax element havinganother name. For example, the LTRP_flag information may be expressed bylong_term_ref_pics_present_flag instead of use_long_term_reference_flag.

When an LTRP in the current sequence is used for inter prediction on thebasis of use_long_term_reference_flag defined in the SPS, each picture(and/or slices) in the current sequence may be used or set as an LTRP.In this case, the video encoder may transmit flag information specifyingwhether a picture to which a current slice belongs is used as an LTRP tothe video decoder. An LTRP_identification number such as LTRP_Id may beallocated to all the pictures used as an LTRP. In this way, whenLTRP-relevant information is transmitted on the basis of theLTRP_identification number, the relevant information may be defined in aslice header. At this time, the LTRP-relevant information transmittedthrough the slice header may be used to construct a long-term referencepicture set. Examples of the LTRP-relevant information defined in theslice header will be described below.

Table 2 shows an example of the LTRP-relevant information defined in theslice header.

TABLE 2 Slice_header( ) { Descriptor  ...  If ((slice_type == I && use_long_term_reference_flag == 1) {   LTRP_flag f(1)   if( LTRP_flag == 1 ) {    LTRP_Id ue(v)    log2_OLC_msb_minus4 ue(v)    OLC_cnt ue(v)  }  }  ... }

In the example shown in Table 2, slice type may specify a slice type ofa current slice. LTRP_flag may specify whether a current picture towhich the current slice belongs is used as an LTRP. For example, whenLTRP_flag equal 0, LTRP_flag may specify that the current picture is notused as an LTRP. When LTRP_flag equal 1, LTRP_flag may specify that thecurrent picture is used as an LTRP. Referring to Table 2, LTRP_flag maybe transmitted when the current slice is an I slice (or the currentpicture is an I picture) and an LTRP is present in the current sequence,that is, when an LTRP in the current sequence is used for interprediction.

When LTRP_flag equal 1, that is, when the current picture is used as anLTRP, additional information relevant to the LTRP may be transmittedthrough the slice header. Referring to Table 2, when the current pictureis used as an LTRP, LTRP_Id, log 2_OLC_msb_minus4, and OLC_cnt may betransmitted in a state where it is included in the slice head.

Here, LTRP_Id may specify an LTRP_identification number of the currentpicture. At this time, two or more LTRPS having the same LTRP_Id may notbe present in the DPB, and only one LTRP for one value of LTRP_Id may bepresent in the DPB. For example, an LTRP having the same value ofLTRP_Id allocated to the current picture may be present already in theDPB. In this case, the LTRP having the same value of LTRP_id presentalready in the DPB before the current picture is stored in the DPB maybe marked as “unused for reference”.

In the example shown in Table 2, when LTRP_flag equal 1, information forderiving an output latency count (OLC) of the current picture used as anLTRP in addition to LTRP_Id may be transmitted through the slice header.Here, the OLC may specify how long the LTRP should be stored in the DPB,that is, the time in which the LTRP should be stored in the DPB.

OLC-relevant information defined in the slice header and transmitted tothe video decoder may be least significant bit (LSB) information andmost significant bit (MSB) information corresponding to the OLC. Here,the LSB may specify at least one bit located at the lowest position in abit sequence of binary numerals specifying the OLC. At this time, theMSB may correspond to a difference between the OLC and the LSB and mayinclude upper bits other than the LSB in the bit sequence of binarynumerals specifying the OLC. The LSB and the MSB may be determined bythe video encoder. When the LSB and the MSB are used, the amount of bitstransmitted from the video encoder to the video decoder may be reducedin comparison with a case where the OLC itself is transmitted.

In the example shown in Table 2, log 2_OLC_msb_minus4 may correspond toa value obtained by subtracting 4 from the number of bits used or setfor the MSB (for example, OLC_msb) of the OLC. That is, log2_OLC_msb_minus4+4 may specify the number of bits used or set for theMSB of the OLC. When log 2_OLC_msb_minus4 is transmitted, the videodecoder may derive the MSB value of the OLC using Expression 1.

OLC_(—) msb=2^(log2) ^(—) ^(OLC) ^(—) ^(msb) ^(—)^(minus+4)  [Expression 1]

In the example shown in Table 2, OLC_cnt may specify the LSBcorresponding to the OLC of the current picture used as an LTRP. WhenOLC_msb and OLC_cnt are determined, the OLC of the current picture usedas an LTRP may be derived using Expression 2.

OLC(CurrPic)=OLC_msb+OLC_cnt  [Expression 2]

Here, OLC(CuurrPic) may specify the OLC of the current picture used asan LTRP.

The LTRP to which the OLC is allocated may be deleted from the memory(for example, the DPB) when the time corresponding to the OLC elapses.Therefore, a picture which is already deleted and which is not availableas a reference picture for inter prediction of the current picture maybe present in the LTRPs stored in the memory before the current pictureis decoded. Such an LTRP may be marked as “unused for reference”depending on a predetermined condition. For example, the LTRP may bemarked as “unused for reference” when the condition of Expression 3 issatisfied.

POC(LTRP)+OLC(LTRP)<=POC(CurrPic)  [Expression 3]

Here, POC(LTRP) indicates the POC of the LTRP. OLC(LTRP) may be an OLCspecifying how long the LTRP is stored in the DPB.

Table 3 shows another example of the LTRP-relevant information definedin the slice header.

TABLE 3 Slice_header( ) { Descriptor  ...  If ((slice_type == I && use_long_term_reference_flag == 1) {   LTRP_flag f(1)   if( LTRP_flag == 1 ) {    LTRP_Id ue(v)   }  }  ... }

Referring to Table 3, LTRP_flag and LTRP_Id may be defined in a sliceheader. That is, in the example shown in Table 3, flag informationspecifying whether the current picture to which the current slicebelongs is used as an LTRP and information on the LTRP_identificationnumber of the current picture may be transmitted to the video decoder ina state where it is included in the slice header, similarly to theexample shown in Table 2. However, the information on the OLC of thecurrent picture used as an LTRP may not be included in the slice header.In this case, the information on the OLC may not be transmitted to thevideo decoder. LTRP_flag and LTRP_Id are described above in the exampleshown in Table 2, and thus will not be specifically described herein.

Table 4 shows another example of the LTRP-relevant information definedin the slice header.

TABLE 4 Slice_header( ) { Descriptor  ...  If ((nal_ref_flag == 1 && use_long_term_reference_flag == 1) {   LTRP_flag f(1)   if( LTRP_flag == 1 ) {    LTRP_Id ue(v)    log2_OLC_msb_minus4 ue(v)    OLC_cnt ue(v)  }  }  ... }

Referring to Table 4, LTRP-flag, LTRP_Id, log 2_OLC_msbminus4, andOLC_cnt may be defined in a slice header. That is, in the example shownin Table 4, flag information specifying whether the current picture towhich the current slice belongs is used as an LTRP, information on theLTRP_identification number of the current picture, and information onthe OLC of the current picture may be transmitted to the video decoderin a state where it is included in the slice header, similarly to theexample shown in Table 2.

However, in the example shown in Table 4, unlike Table 2, all the slices(and/or pictures) as well as the I slice (and/or I picture) may be usedas an LTRP. Therefore, LTRP_flag and LTRP_Id may be transmittedregardless of whether the current slice is an I slice. That is, in theexample shown in Table 2, LTRP_Id may be allocated to the currentpicture regardless of whether the current slice is an I slice.

LTRP-flag, LTRP_Id, log 2_OLC_msb_minus4, and OLC_cnt are describedabove in the example shown in Table 2, and thus will not be specificallydescribed herein.

Table 5 shows another example of the LTRP-relevant information definedin the slice header.

TABLE 5 Slice_header( ) { Descriptor  ...  If ((nal_ref_flag == 1 && use_long_term_reference_flag == 1) {   LTRP_flag f(1)   if( LTRP_flag == 1) {    LTRP_Id ue(v)   }  }  ... }

Referring to Table 5, LTRP_flag and LTRP_Id may be defined in a sliceheader. That is, in the example shown in Table 5, flag informationspecifying whether the current picture to which the current slicebelongs is used as an LTRP and information on the LTRP_identificationnumber of the current picture may be transmitted to the video decoder ina state where it is included in the slice header, similarly to theexample shown in Table 4. However, the information on the OLC of thecurrent picture used as an LTRP may not be included in the slice header.In this case, the information on the OLC may not be transmitted to thevideo decoder. LTRP_flag and LTRP_Id are described above in the exampleshown in Table 4, and thus will not be specifically described herein.

When LTRP_Id is allocated to the respective pictures used as an LTRP inany example shown in Tables 2 to 5, the video decoder may determineLTRPs constituting a long-term reference picture set on the basis of thevalues of LTRP_Id allocated to the pictures.

LTRP_Id may be usefully used particularly in the course of explicitreference picture signaling (hereinafter, referred to as ERPS). When thePOC of an LTRP is directly transmitted in the course of ERPS or a POCdifference between the current picture and the LTRP is transmitted, theamount of information to be transmitted may be very large. When theLTRP-relevant information is transmitted on the basis of LTRP_Id, theamount of information to be transmitted may be reduced and theabove-mentioned problem may be solved.

When LTRP_Id allocated to the respective pictures (and/or slices) istransmitted as in the examples shown in Tables 2 to 5, information forconstructing a long-term reference picture set may be defined on thebasis of LTRP_Id in the slice header of a slice to be decoded. When theERPS method is used, the LTRP-relevant information should be signaledindependently of the STRP-relevant information. Therefore, LTRP-relevantsyntax elements for constructing the long-term reference picture set maybe defined independently of STRP-relevant syntax elements in the sliceheader. Table 6 shows an example of a sliced header includinginformation for constructing a long-term reference picture set based onLTRP_Id.

TABLE 6 Slice_header( ) { Descriptor  ... If ((slice_type == P || slice_type == B) &&explicit_ref_pic_signaling_flag == 1) {   ref_pic_pattern_idc ue(v)  if( ref_pic_pattern_type == 0 ) {    number_of_ref_frame_minus1 ue(v)   for( i = 0; i < number_of_ref_pic_minus1 + 1; i++ ) {    sign_bit_of_ref_frame[i] f(1)     abs_ref_frame[ i ] ue(v)    }   }else {    for( i = 0; i < ref_pattern [ref_pic_pattern_idc −1][0] + 1;i++ ) {     Sign_bit_ref_frame[ i ] =      sign_bit_of_ref_pattern[ref_pic_pattern_idc −1][i +       1]    abs_ref_frame[ i ]=       abs_ref_pattern[ref_pic_pattern_idc−1][i + 1]    }   }   if (use_long_term_reference_flag == 1) {    num_used_long_term_pics ue(v)     if (num_used_long_term_pics > 0) {      for (i = 0; i < num_used_long_term_pics; i++) {        used_LTRP_Id [i] ue(v)       }     }   }  }  ... }

Table 6 shows a method of signaling LTRP-relevant information throughthe slice header when the ERPS method is used. num_used_long_term_picsand used_LTRP_Id[i] may be defined in the slice header shown in Table 6.That is, in the example shown in Table 6, num_used_long_term_pics andused_LTRP_Id[i] may be transmitted to the video decoder in a state wherethey are included in the slice header. Referring to Table 6, num_usedlong_term_pics and used_LTRP_Id[i] may be transmitted to the videodecoder when use_long_term_reference_flag described in the example shownin Table 1 equal 1, that is, when an LTRP in the current sequence isused for inter prediction.

Here, num_used_long_term_pics indicates the number of LTRPs necessaryfor inter prediction of the current picture. In addition,used_LTRP_Id[i] (where i is an integer of 0 or greater) may specify theLTRP_identification number of the i-th (or (i+1)-th) LTRP used for interprediction of the current picture. The video decoder may use pictures(and/or slice) indicated by used_LTRP_Id[i] as LTRPs of the currentpicture and/or the current slice). For example, the long-term referencepicture set in this case may include the pictures indicated byused_LTRP_Id[i]. At this time, the video decoder may construct thelong-term reference picture set by allocating the pictures indicated byused_LTRP_Id[i] in the ascending order of i magnitudes. The course ofconstructing a reference picture list on the basis of the long-termreference picture set is described above with reference to FIG. 4 andthus will not be repeatedly described.

In the example shown in Table 6, the LTRPs used for inter prediction ofthe current picture are indicated by used_LTRP_Id[i], but the presentinvention is not limited to this configuration. For example, a pictureused asn an LTRP may be indicated by the POC of the LTRP or the LSB andthe MSB corresponding to the POC. Specific examples thereof will bedescribed later.

The slice header including information (for example, used_LTRP_Id[i])for constructing the long-term reference picture set on the basis ofLTRP_Id is not limited to the above-mentioned examples. In the exampleshown in Table 6, syntax elements other than the syntax elements (forexample, use_long_term_reference_flag, num_used_long_term_pics, andused_LTRP_Id[i]) for signaling the LTRP-relevant information based onthe ERPS method may be applied in different ways depending onimplementation and/or necessity.

On the other hand, in the above-mentioned examples, when an LTRP in thecurrent sequence is used for inter prediction, the LTRP-relevantinformation other than use_long_term_reference_flag is transmittedthrough the slice header. However, a part of the LTRP-relevantinformation may be transmitted to the video decoder in a state where itis included in a picture parameter set (PPS).

Table 7 shows an example of the LTRP-relevant information defined in aPPS.

TABLE 7 pic_parameter_set_rbsp ( ) { Descriptor  ...  if(use_long_term_reference_flag) {   num_long_term_pics_list ue(v)   If(num_long_term_pics_list > 0) {    ltrp_poc_len_minus4 ue(v)    for (i =0; i < num_long_term_pics_list; i++) {     ltrp_poc [ i ] u(v)    }   } }  ... }

In the example shown in Table 7, num_long_term_pics_list may specify thenumber of slices referring to the current PPS, that is, the number ofcandidate LTRPs which can be used in the slices included in the currentpicture. Referring to Table 7, num_long_term_pics_list may betransmitted to the video decoder in a state where it is included in theslice header when use_long_term_reference_flag equal 1.

In the following description, a candidate LTRP which can be used in aslice referring to the current PPS, that is, a candidate LTRP specifiedin the PPS, is referred to as “PPS candidate LTRP” for the purpose ofconvenience of explanation.

ltrp_poc_len_minus4 may specify a value obtained by subtracting 4 fromthe number of bits necessary for expressing a syntax elementltrp_poc[i]. That is, “ltrp_poc_len_minus4+4” may specify the number ofbits necessary for expressing the syntax element ltrp_poc[i].ltrp_poc[i] (where i is an integer of 0 or greater) may specify the POC(for example, the absolute value of the POC) of the i-th (or (i+1)-th)PPS candidate LTRP out of the PPS candidate LTRPs. That is, the picturesused as the PPS candidate LTRP may be indicated by ltrp_poc[i]. Forexample, the number of bits for expressing ltrp_poc[i] may be“ltrp_poc_len_minus4+4”. Referring to Table 7, ltrp_poc_len_minus4 andltrp_poc[i] may be transmitted to the video decoder in a state wherethey are included in the slice header when the value ofnum_long_term_pics_list equal 0, that is, when the number of PPScandidate LTRPs is greater than 0.

As in the example shown in Table 7, when the information on the PPScandidate LTRP is transmitted through the PPS, the LTRP to be used toconstruct a long-term reference picture set may be determined on thebasis of the PPS candidate LTRP at the level of slice. Accordingly,information for determining the LTRP to be used to construct a long-termreference picture set on the basis of the PPS candidate LTRP may bedefined in the slice header. Table 8 shows an example of information tobe transmitted through a slice header when the PPS candidate LTRP isdetermined as in the example shown in Table 7.

TABLE 8 slice_header( ) { Descriptor  ... if(use_long_term_reference_flag) {   num_long_term_pics ue(v)   for( i= 0; i < num_long_term_pics; i++ ) {    ltrp_idx [ i ] ue(v)   used_by_curr_pic_lt_flag[ i ] u(1)   }  }  ... }

In the slice header shown in Table 8, num_long_term_pics, ltrp_idx[i],and used_by_curr_pic_h_flag[i] may be defined. Referring to Table 8,num_long_term_pics, and used_by_curr_pic_lt_flag[i] may be transmittedto the video decoder when the value of use_long_term_reference_flagdescribed in conjunction with the example shown in Table 1 equal 1, thatis, when the LTRP in the current sequence is used to inter prediction.

num_long_term_pics may specify the number of LTRPs to be included in thelong-term reference picture set of the current picturenum_long_term_pics may have a value in a range of “0” to“max_num_ref_frames−NumNegativePics[StRpsIdx]−NumPositivePics[StRpsIdx]”.Here, max_num_ref_frames may specify the maximum number of referencepictures to be stored in the DPB. NumNegativePics[StRpsIdx] may specifythe number of forward short-term reference pictures andNumPositivePics[StRpsIdx] may specify the number of backward short-termreference pictures. num_long_term_pics may have a value which isincluded in the above-mentioned range and which is included in a rangeof “0” to “num_long_term_pics_list” (or “num_long_term_pics_list−1”). Asdescribed above, num_long_term_pics_list may specify the number of PPScandidate LTRPs. When num_long_term_pics is not present, the value ofnum_long_term_pics may be considered or estimated to be 0.

ltrp_idx[i] (where i is an integer of 0 or greater) may specify theindex of the i-th (or (i+1)-th) LTRP used for inter prediction of thecurrent picture (and/or the current slice) out of the indices of the PPScandidate LTRPs. Accordingly, the video decoder may determine the PPScandidate LTRPs to be used as the LTRP of the current picture on thebasis of ltrp_idx[i] transmitted through the slice header. In this case,since the information on the POC can be indexed, it is possible toreduce the amount of information transmitted from the video encoder tothe video decoder.

Here, the index of a PPS candidate LTRP may mean i in the syntax elementltrp_idx[i] defined in the PPS shown in Table 7. Therefore, the POC ofthe i-th (or (i+1)-th) LTRP used for inter prediction of the currentpicture (and/or the current slice) in this case may beltrp_poc[ltrp_idx[i]].

used_by_curr_pic_h_flag[i] may specify whether the i-th LTRP indicatedby the ltrp_idx[i] is used as a referene picture for the currentpicture. For example, when ltrp_idx[i] equal 1, the i-th LTRP may beused as a reference picture for the current picture. However, whenltrp_idx[i] equal 0, the i-th LTRP may not be used as a referencepicture for the current picture.

In the example shown in Table 8, the video decoder may use the LTRPs inwhich used_by_curr_pic_lt_flag[i] is set o 1 out of the LTRPs indicatedby ltrp_idx[i] to construct a long-term reference picture set. Forexample, the video decoder may construct the long-term reference pictureset by allocating the LTRPs in which used_by_curr_pic_lt_flag[i] equal 1out of the LTRPs indicated by ltrp_idx[i] in the ascending order ofmagnitudes of i. The course of constructing a reference picture list onthe basis of the long-term reference picture set is described above withreference to FIG. 4 and thus will not be repeatedly described herein.

On the other hand, in the course of constructing a reference picture setand/or constructing a reference picture list, each reference picture maybe marked as “unused for reference”, “used for short-term reference”, or“used for long-term reference”. For example, a picture not used toconstruct a reference picture set may be marked as “unused forreference”. An STRP constituting a short-term reference picture set maybe marked as “used for short-term reference”, an LTRP constituting along-term reference picture set may be marked as “used for long-termreference”.

As described above, ltrp_poc[i] specifying the POC (for example, theabsolute value of the POC) of a PPS candidate LTRP may be defined in thePPS. Accordingly, when the video encoder determines that an arbitraryreference picture should be set as a new LTRP, a new PPS including thePOC information of the PPS candidate LTRP determined on the basis of thenew LTRP should be transmitted to the video decoder. In this case, forexample, the POC of the new LTRP may be included and considered in atable for mapping the POC information of the PPS candidate LTRP.

At this time, an LTRP not indicated by ltrp_idx[i] of the slice header(an LTRP not included in the slice header) may be present in the PPTcandidate LTRPs indicated by ltrp_poc[i] defined in the PPS (LTRPsincluded in the PPS). At this time, an LTRP which belongs to the PPScandidate LTRPs included in the PPS and which is not included in theslice header may be marked as “unused for reference”.

When an LTRP in the current bit stream is includes the slice header ofthe current picture but is not included in a slice header of a picturehaving a POC smaller than that of the current picture, the correspondingbit stream may be considered as illegal.

On the other hand, as described in conjunction with the example shown inTable 1, the LTRP flag information in the unit of SPS defined in the SPSmay be expressed by long_term_ref_pics_present_flag instead ofuse_long_term_reference_flag. In this case, the LTRP flag informationdefined in the SPS may be expressed as in the example shown in Table 9.

TABLE 9 seq_parameter_set_rbsp( ) { Descriptor   ...  long_term_ref_pics_present_flag u(1)   ... }

In the example shown in Table 9, long_term_ref_pics_present_flag mayspecify whether an LTRP in a currently-encoded video sequence is usedfor inter prediction of at least one encoded picture, that is, whetheran LTRP in the current sequence is used. For example, whenlong_term_ref_pics_present_flag equal to 0, the flag may specify that anLTRP in the currently-encoded video sequence is not used for interprediction of any encoded picture. In this case, a picture used as anLTRP may not be present in the currently-encoded video sequence. Whenlong_term_ref_pics_present_flag equal to 1, the flag may specify that anLTRP in the currently-encoded video sequence is used for interprediction of at least one encoded picture.

In the example shown in Table 9, when long_term_ref_pics_present_flagindicates that an LTRP in the currently-encoded video sequence is notused for inter prediction, the video encoder may not transmit long-termreference picture-relevant information other thanlong_term_ref_pics_present_flag to the video decoder. Accordingly, thevideo decoder may neither receive nor process the long-term referencepicture-relevant information other than long_term_ref_pics_present_flag.

When an LTRP in the current sequence is used for inter prediction on thebasis of long_term_ref_pics_present_flag defined in the SPS of Table 9,other long-term reference picture set-relevant information may bedefined in the sliced header. In this case, the video encoder maytransmit the long-term reference picture-relevant information other thanlong_term_ref_pics_present_flag to the video decoder through the sliceheader. Table 10 shows an example of the long-term referencepicture-relevant information transmitted through a slice header whenlong_term_ref_pics_present_flag is transmitted through the SPS as inTable 9.

TABLE 10 slice_header( ) { Descriptor  ...   if(long_term_ref_pics_present_flag ) {    num_long_term_pics ue(v)    for(i = 0; i < num_long_term_pics; i++ ) {     poc_lsb_lt[ i ] u(v)    delta_poc_msb_present_flag[ i ] u(1)     if(delta_poc_msb_present_flag[ i ] )      delta_poc_msb_cycle_lt[ i ] ue(v)    used_by_curr_pic_lt_flag[ i ] u(1)    }   }  ... }

In the slice header shown in Table 10, num_long_term_pics,poc_lsb_lt[i], delta_poc_msb_present_flag[i], delta_poc_msb_cycle_lt[i],and used_by_curr_pic_lt_flag[i] may be defined. In the example shown inTable 10, when long_term_ref_pics_present_flag equal to 1, that is, whenan LTRP in the current sequence is used for inter prediction,num_long_term_pics, poc_lsb_lt[i], delta_poc_msb_present_flag[i],delta_poc_msb_cycle_lt[i], and used_by_curr_pic_lt_flag[i] may betransmitted to the video decoder in a state where they are included inthe slice header.

num_long_term_pics may specify the number of LTRPs to be included in along-term reference picture set of the current picture. Here, the LTRPsto be included in the long-term reference picture set may be picturesstored in the DPB before the current slice is decoded.

In the example shown in Table 10, the POC of the LTRPs to be included inthe long-term reference picture set may be specified on the basis ofinformation relevant to the LSB and the MSB corresponding to the POC.Here, the LSB relevant information (poc_lsb_lt[i]) and the MSB relevantinformation (delta_poc_msb_present_flag[i] anddelta_poc_msb_cycle_lt[i]) may be transmitted from the video encoder tothe video decoder. In the following description, for the purpose ofconvenience of explanation, the LSB of a POC is referred to as a POCLSB, and the MSB of a POC is referred to as a POC MSB.

The POC LSB may specify at least one bit located at the lowest positionin a bit sequence of binary numerals specifying the POC of the referencepictures. The POC MSB may correspond to the difference between the POCand the LSB and may include higher bits other than the POC LSB in thebit sequence of binary numerals specifying the POC. Here, the POC LSBand the POC MSB may be determined by the video encoder and may betransmitted to the video decoder. When the POC LSB and the POC MSB areused, it is possible to reduce the amount of bits transmitted from thevideo encoder to the video decoder, compared with a case where the POCitself is transmitted.

The video encoder may transmit information on the maximum value of theLSB corresponding to the POC to the video decoder. At this time, theinformation may specify the maximum number of bits used for the LSB. Forexample, the information on the maximum value of the LSB may betransmitted to the video decoder in a state where it is included in theSPS, and may be indicated by log 2_max_pic_order_cnt_lsb_minus4. Here,log 2_max_pic_order_cnt_lsb_minus4 may correspond to a value obtained bysubtracting 4 from the maximum number of bits used for the LSB. When log2_max_pic_order_cnt_lsb_minus4 is received, the video decoder may derivethe maximum value of the LSB of the POC using Expression 4.

MaxPicOrderCntLsb=2^((log2) ^(—) ^(max) ^(—) ^(pic) ^(—) ^(order) ^(—)^(cnt) ^(—) ^(lsb) ^(—) ^(minus4+4))  [Expression 4]

Here, MaxPicOrderCntLsb may mean the maximum value of the LSBcorresponding to the POC.

poc_lsb_lt[i] (where i is an integer of 0 or greater) may specify thePOC LSB of the i-th candidate LTRP to be included in the long-termreference picture set of the current picture. That is, the POC LSB ofthe LTRP may be specified by poc_lsb_lt[i].

delta_poc_msb_present_flag[i] may specify whetherdelta_poc_msb_cycle_lt[i] is present. That is, the video encoder maytransmit flag information specifying whether MSB information fordistinguish the POC MSB of the LTRP is present to the video decoder. Forexample, when there is no possibility that two or more LTRPs having thesame POC LSB value will be present, the flag may be set to 0. Otherwise,the flag may be set to 1.

Referring to Table 10, when delta_poc_msb_present_flag[i] equal to 1,that is, when the MSB information for distinguishing the POC MSB of theLTRP is present, delta_poc_msb_cycle_lt[i] may be transmitted to thevideo decoder in a state where it is included in the slice header.delta_poc_msb_cycle_lt[i] may be used to determine the MSB of the POC ofthe i-th candidate LTRP to be included in the long-term referencepicture set of the current picture. That is, the MSB of the POC of thei-th candidate LTRP may be determined on the basis ofdelta_poc_msb_cycle_lt[i] using a predetermined calculation.

When the POC LSB and/or the POC MSB is determined through poc_lsb_lt[i],delta_poc_msb_present_flag[i], and/or delta_poc_msb_cycle_lt[i], thevideo decoder may derive the POC of the i-th candidate LTRP to beincluded in the long-term reference picture set of the current pictureon the basis thereof.

In the example shown in Table 10, used_by_curr_pic_lt_flag[i] mayspecify whether the i-th candidate LTRP is used as a reference pictureof the current picture (and/or the current slice). For example, whenused_by_curr_pic_lt_flag[i] equal to 0, the i-th candidate LTRP may notbe used as a reference picture of the current picture (and/or thecurrent slice). At this time, the i-th candidate LTRP may not beincluded in the long-term reference picture set used to construct areference picture list.

In the example shown in Table 10, the video decoder may use the LTRPsindicated by the POC derived on the basis of the POC LSB and/or the POCMSB to construct the long-term reference picture set. For example, thevideo decoder may construct the long-term reference picture set byallocating the LTRPs other than the LTRP in whichused_by_curr_pic_lt_flag[i] equal 0 out of the LTRPs derived on thebasis of the POC LSB and/or the POC MSB in the ascending order ofmagnitudes of i. The process of constructing a reference picture list onthe basis of the long-term reference picture set is described above withreference to FIG. 4 and description thereof will not be repeated herein.

In the examples shown in Tables 9 and 10, when an LTRP is used for interprediction, the LTRPs stored in the DPB may be signaled using the POCLSB (for example, poc_lsb_lt[i]) through the slice header. However, whenthe POC LSB is transmitted for each slice, the same POC LSB value in onesequence may be repeatedly transmitted and thus there may be a problemin signaling efficiency of LTRP-relevant information.

In the example shown in Table 10, since u(v) is allocated as adescriptor to poc_lsb_lt[i], a “fixed length encoding” method based on afixed number of bits v (where v is a natural number) may be applied topoc_lsb_lt[i]. In this case, the number of bits used to encodepoc_lsb_lt[i] in the current sequence may not be changed. Here v may bedetermined by log 2_max_pic_order_cnt_lsb_minus4 defined in the SPS. Asdescribed above, “log 2_max_pic_order_cnt_lsb_minus4+4” may indicate themaximum number of bits used for the LSB. Therefore, the number of bitsused to encode poc_lsb_lt[i] may correspond to the maximum number ofbits of the POC LSB.

However, since an LTRP has a feature that it is not changed in a longperiod, the number of the POC LSB values of the LTRPs used for theactual encoding/decoding process may not be large. It may be inefficientthat the maximum number of bits is allocated to signal the POC LSB ofthe LTRP in this situation.

Therefore, an LTRP signaling method for solving the problem occurring inthe examples shown in Tables 9 and 10 may be provided. Examples of theLTRP signaling method for solving the above-mentioned problem will bedescribed below.

As described above, an LTRP has a feature that it is stored in the DPBand is not changed in a very long period. Therefore, it may be redundantto transmit the same POC LSB value for each slice header. Accordingly,in order to improve signaling efficiency, the POC LSB values having ahigh possibility to be used as the POC LSB may be transmitted to thevideo decoder in a state where they are included in the SPS.

In the following description, the POC LSB included through the SPS isreferred to as “SPS POC LSB”. Here, plural POC LSBs to be transmittedthrough the SPS may be considered to construct one set and this set maybe referred to as an “SPS POC LSB set” or an “SPS LSB set” in thefollowing description. Since the POC LSB to be transmitted through theSPS corresponds to the LTRP, the POC LSB to be transmitted through theSPS may be referred to as an “SPS candidate LTRP” in the followingdescription. Plural SPS candidate LTRPs to be transmitted through theSPS may be considered to construct an “SPS candidate LTRP list”.

When the POC LSB of an LTRP to be signaled through the slice is the sameas the SPS POC LSB to be transmitted through the SPS, an indexspecifying the SPS POC LSB may be transmitted through the slice insteadof the POC LSB of the LTRP. Therefore, in this case, it is possible toreduce the amount of bits necessary for signaling the POC LSB of theLTRP.

Table 11 shows an example of an SPS including information on the SPS POCLSB.

TABLE 11 seq_parameter_set_rbsp( ) { Descriptor  ... long_term_ref_pics_present_flag u(1)  if(long_term_ref_pics_present_flag ) {   num_long_term_ref_pics_sps ue(v)  for( i = 0; i < num_long_term_ref_pics_sps; i++ )   lt_ref_pic_poc_lsb_sps[ i ] u(v)  }  ... }

Referring to Table 11, long_term_ref_pics_present_flag may be defined inthe SPS. That is, the video encoder may transmitlong_term_ref_pics_present_flag to the video decoder through the SPS.This flag is the same as described above in conjunction with the exampleshown in Table 9 and specific description thereof will not be repeated.

When long_term_ref_pics_present_flag equal to 1, that is, when an LTRPin the current sequence is used for inter prediction,num_long_term_ref_pics_sps and lt_ref_pic_poc_lsb_sps[i] may betransmitted to the video decoder in a state where they are included inthe SPS.

num_long_term_ref_pics_sps may specify the number of SPS candidate LTRPsto be specified or transmitted through the SPS. That is, the videoencoder may transmit information on the number of POC LSBs having a highpossibility to be used as the POC LSB of the LTRP to the video decoderthrough the SPS. Here, for example, num_long_term_ref_pics_sps may havea value in a range of 0 to 32.

When num_long_term_ref_pics_sps equal a value greater than 0, that is,when the number of SPS candidate LTRPs to be transmitted through the SPSis greater than 0, lt_ref_pic_poc_lsb_sps[i] may be transmitted to thevideo decoder in a state where it is included in the SPS.lt_ref_pic_poc_lsb_sps[i] (where i is an integer of 0 or greater) mayspecify the POC LSB of the i-th SPS candidate LTRP to be specified ortransmitted through the SPS. This may mean that the POC LSB set istransmitted through the SPS. Here, the number of bits necessary forexpressing lt_ref_pic_poc_lsb_sps[i] may be “log2_max_pic_order_cnt_lsb_minus4+4”. log 2_max_pic_order_cnt_lsb_minus4 isdescribed above and thus specific description thereof will not berepeated herein.

When the information on the SPS POC LSB is transmitted through the SPSas described above in conjunction with the example shown in Table 11,the LTRP-relevant information defined in the slice header may beexpressed, for example, by Table 12.

TABLE 12 slice_header( ) { Descriptor  ...  if(long_term_ref_pics_present_flag ) {   num_long_term_pics ue(v)   if(num_long_term_ref_pics_sps > 0 )    num_long_term_sps ue(v)  for( i = 0; i < num_long_term_sps + num_long_term_pics; i++ ) {    if( i < num_long_term_sps )     lt_idx_sps[ i ] u(v)    else    poc_lsb_lt[ i ] u(v)    delta_poc_msb_present_flag[ i ] u(1)    if(delta_poc_msb_present_flag[ i ] )     delta_poc_msb_cycle_lt[ i ] ue(v)   used_by_curr_pic_lt_flag[ i ] u(1)   }  }  ... }

When long_term_ref_pics_present_flag equal to 1, that is, when an LTRPin the current sequence is used for inter prediction, the LTRP-relevantinformation may be transmitted through the slice header.

In the example shown in Table 12, num_long_term_sps may specify thenumber of SPS candidate LTRPs to be included in the long-term referencepicture set of the current picture (and/or the current slice). That is,when the number of SPS POC LSBs to be transmitted through the SPS isgreater than 0, the video encoder may transmit information on the numberof SPS candidate LTRPs to be included in the long-term reference pictureset of the current picture to the video decoder. In this case, the SPScandidate LTRPs to be included in the long-term reference picture set ofthe current picture (and/or the current slice) may be pictures stored inthe DPB before the current slice is decoded.

When num_long_term_sps is not present, the value of num_long_term_spsmay be inferred or estimated to be 0. num_long_term_sps may have a valuein a range of “0” to “Min(num_long_term_ref_pics_sps,max_dec_pic_buffering[max_temporal_layers_minus1]−NumNegativePics[StRpsIdx]−NumPositivePics[StRpsIdx]−num_long_term_pics)”.Here, max_dec_pic_buffering[max_temporal_layers_minus1] may specify themaximum number of reference pictures to be stored in the DPB.num_long_term_ref_pics_sps, NumNegativePics[StRpsIdx], andNumPositivePics[StRpsIdx] are described above and thus descriptionthereof will not be repeated herein.

In the example shown in Table 12, num_long_term_pics may specify thenumber of LTRPs which are directly signaled or specified through thecurrent slice header as the LTRPs to be included in the long-termreference picture set of the current picture (and/or the current slice).That is, the video encoder may transmit the information on the number ofLTRPs other than the SPS candidate LTRPs (or not referring the SPScandidate LTRPs) as the LTRPs to be included in the long-term referencepicture set of the current picture (and/or the current slice) to thevideo decoder. In this case, the LTRPs to be included in the long-termreference picture set of the current picture (and/or the current slice)may be pictures stored before the current slice is decoded.

When num_long_term_pics is not present, the value of num_long_term_picsmay be inferred or estimated to be 0. num_long_term_pics may have avalue in a range of “0” to“max_dec_pic_buffering[max_temporal_layers_minus1]−NumNegativePics[StRpsIdx]−NumPositivePics[StRpsIdx]−num_long_term_sps”.

On the other hand, as described above, when the POC LSB of the LTRP tobe signaled through the slice is the same as the SPS POC LSB to betransmitted through the SPS, an index specifying the SPS POC LSB may betransmitted through the slice instead of the POC LSB of the LTRP. Thatis, a list of LTRPs determined on the basis of the SPS POC LSB may betransmitted through the slice header.

Referring to Table 12, lt_idx_sps[i] (where i is an integer of 0 orgreater) may indicate an index of a picture (SPS candidate LTRP) to beincluded as the i-th LTRP in the long-term reference picture set of thecurrent picture among the pictures (SPS candidate LTRPs) included in theSPS candidate LTRP list. That is, lt_idx_sps[i] may specify an index inthe SPS candidate LTRP list for identifying a picture to be included inthe long-term reference picture set of the current picture. Accordingly,the video decoder may determine the SPS candidate LTRPs to be used asthe LTRPs of the current picture on the basis of lt_idx_sps[i]transmitted through the slice header. In this case, since theinformation on the POC LSB can be indexed, it is possible to reduce theamount of information transmitted from the video encoder to the videodecoder.

The index of an SPS candidate LTRP may mean i in a syntax elementlt_ref_pic_poc_lsb_sps[i] defined in the SPS of Table 11. Accordingly,in this case, the POC LSB of the i-th candidate LTRP to be included inthe long-term reference picture set of the current picture (and/or thecurrent slice) may be lt_ref_pic_poc_lsb_sps[lt_idx_sps[i]].

lt_idx_sps[i] may have a value in a range of “0” to“num_long_term_ref_pics_sps−1”. Here, the number of bits used to expresslt_idx_sps[i] may be “ceil(log 2(num_long_term_ref_pics_sps))”. Here,“ceil(X)” may be a minimum integer equal to or greater than X.

When the POC LSB of an LTRP to be signaled through the slice is not thesame as the SPS POC LSB to be transmitted through the SPS, the POC LSBof the LTRP may be directly transmitted through the slice. That is, alist of LTRPs not determined on the basis of the SPS POC LSB may beadditionally transmitted through the slice header.

Referring to Table 12, poc_lsb_lt[i] may specify the POC LSB of the i-thcandidate LTRP to be included in the long-term reference picture set ofthe current picture. That is, the video encoder may directly orexplicitly transmit the POC LSB of the LTRP to be included in thelong-term reference picture set of the current picture to the videodecoder through the slice header.

poc_lsb_lt[i] may have a value in a range of “0” to “MaxPicOrderCntLsb”.Here, the number of bits used to express poc_lsb_lt[i] may be “log2_max_pic_order_cnt_lsb_minus4+4”.

In the example shown in Table 12, the POC LSBs of some LTRPs(hereinafter, referred to as first candidate LTRPs) out of the candidateLTRPs to be included in the long-term reference picture set of thecurrent picture may be determined on the basis of the SPS POC LSB. ThePOC LSBs of the other LTRPs (hereinafter, referred to as secondcandidate LTRPs) out of the candidate LTRPs to be included in thelong-term reference picture set of the current picture may be determinedon the basis of poc_lsb_lt[i] explicitly transmitted through the sliceheader. At this time, the candidate LTRPs to be included in thelong-term reference picture set of the current picture may be consideredto include the first candidate LTRPs and the second candidate LTRPs.Accordingly, when the POC LSBs of the first candidate LTRPs and the POCLSBs of the second candidate LTRPs are all determined, it may beconsidered that all the POC LSBs of the candidate LTRPs to be includedin the long-term reference picture set of the current picture aredetermined.

Referring to Table 12 again, information for deriving the POC MSBs ofthe candidate LTRPs to be included in the long-term reference pictureset of the current picture may be transmitted through the slice header.The information for deriving the POC MSBs in the slice header mayinclude delta_poc_msb_present_flag[i] and delta_poc_msb_cycle_lt[i].

delta_poc_msb_present_flag[i] may specify whetherdelta_poc_msb_cycle_lt[i] is present. That is, the video encoder maytransmit flag information specifying whether MSB information fordistinguishing the POC MSBs of the LTRPs to the video decoder.

For example, when there is no possibility that two or more LTRPs havingthe same POC LSB value will be present, delta_poc_msb_present_flag[i]may be 0. At this time, this flag may specify thatdelta_poc_msb_cycle_lt[i] is not present. On the contrary, when two ormore reference pictures having the same POC LSB value as poc_lsb_lt[i]are present in the DPB, delta_poc_msb_present_flag[i] may be 1. At thistime, this flag may specify that delta_poc_msb_cycle_lt[i] is present.For example, it is assumed that j has an integer value which is not thesame as i in the range of “0” to “num_long_term_pics−1”. Then, when avalue in which poc_lsb_lt[i] and poc_lsb_lt[j] are equal to each otheris present in possible values of j, delta_poc_msb_present_flag[i] may beset to 1.

Referring to Table 12 again, when delta_poc_msb_present_flag[i] equal 1,delta_poc_msb_cycle_lt[i] may be transmitted to the video decoder in astate where it is included in the slice header.delta_poc_msb_cycle_lt[i] may be used to determine the POC MSB of thei-th candidate LTRP to be included in the long-term reference pictureset of the current picture. That is, the POC MSB of the i-th candidateLTRP may be determined on the basis of delta_poc_msb_cycle_lt[i] using apredetermined calculation.

In the course of deriving the POC MSB, the video decoder may derive avariable DeltaPocMSBCycleLt[i] on the basis of delta_poc_msb_cycle_lt[i]transmitted from the video encoder. At this time, the POC MSB may becalculated on the basis of the value set as DeltaPocMSBCycleLt[i]. Thestep of deriving DeltaPocMSBCycleLt[i] may be expressed, for example, asfollows.

if( i = = 0 | | poc_lsb_lt [ i - 1 ] ! = poc_lsb_lt [ i ] ) DeltaPocMSBCycleLt[ i ] = delta_poc_msb_cycle_lt[i ] else DeltaPocMSBCycleLt[ i ] =   delta_poc_msb_cycle_lt[ i ]+DeltaPocMSBCycleLt[ i - 1 ]

-   -   when the POC LSB and/or the POC MSB is determined using        lt_idx_sps[i], poc_lsb_lt[i], delta_poc_msb_present_flag[i],        and/or delta_poc_msb_cycle_lt[i], the video decoder may derive        the POC of the i-th candidate LTRP to be included in the        long-term reference picture set of the current picture on the        basis thereof.

In the example shown in Table 12, used_by_curr_pic_lt_flag[i] mayspecify whether the i-th candidate LTRP to be included in the long-termreference picture set of the current picture is used as a referencepicture of the current picture (and/or the current slice). For example,when used_by_curr_pic_lt_flag[i] equal 0, the i-th candidate LTRP maynot be used as a reference picture of the current picture (and/or thecurrent slice). At this time, the i-th candidate LTRP may not beincluded in the long-term reference picture set used to construct areference picture list.

According to the example shown in Table 12, the video decoder may usethe LTRPs specified by the POCs derived on the basis of the POC LSBand/or the POC MSB to construct the long-term reference picture set. Forexample, the video decoder may construct the long-term reference pictureset by allocating the LTRPs other than the LTRPs in whichused_by_curr_pic_lt_flag[i] equal 0 out of the candidate LTRPs derivedon the basis of the POC LSB and/or the POC MSB in the ascending order ofmagnitudes of i. The course of constructing a reference picture list onthe basis of the long-term reference picture set is described above withreference to FIG. 4 and description thereof will not be repeated herein.

Table 13 shows another example of the LTRP-relevant information definedin the slice header when the LTRP-relevant information is transmittedthrough the SPS as in the example shown in Table 11.

TABLE 13 slice_header( ) { Descriptor  ...  if(long_term_ref_pics_present_flag ) {   num_long_term_pics ue(v)   if(num_long_term_ref_pics_sps > 0 )    num_long_term_sps ue(v)  for( i = 0; i < num_long_term_sps + num_long_term_pics; i++ ) {    if( i < num_long_term_sps )     lt_idx_sps[ i ] ue(v)    else    poc_lsb_lt[ i ] u(v)    delta_poc_msb_present_flag[ i ] u(1)    if(delta_poc_msb_present_flag[ i ] )     delta_poc_msb_cycle_lt[ i ] ue(v)   used_by_curr_pic_lt_flag[ i ] u(1)   }  }  ... }

In the example shown in Table 12, u(v) is set as a descriptor oflt_idx_sps[i]. Accordingly, a “fixed length encoding” method based on afixed number of bits v may be applied to lt_idx_sps[i]. However, sincean LTRP has a feature that it is not changed in a very long period, itmay be inefficient to apply the fixed length encoding to lt_idx_sps[i].

Therefore, an encoding method of adaptively determining a bit length maybe applied to It_idx_sps[i]. An example of the encoding method ofadaptively determining a bit length may be an exponential golombencoding method.

Referring to Table 13, ue(v) may be set as a descriptor oflt_idx_sps[i]. That is, lt_idx_sps[i] may be encoded and transmittedusing ue(v) through the use of slice header. ue(v) may specify a syntaxelement to be encoded on the basis of the exponential golomb encodingmethod. Accordingly, in the example shown in Table 13, lt_idx_sps[i] maybe encoded on the basis of an exponential golomb code and may betransmitted to the video decoder. Here, the exponential golomb code maybe, for example, a 0-th order exponential golomb code.

When the fixed length encoding method is applied as in the example shownin Table 12, the number of bits used to express lt_idx_sps[i] may be“ceil(log 2(num_long_term_ref_pics_sps))”. However, when the exponentialgolomb encoding method is used, the bit length may be variablydetermined on the basis of the exponential golomb code and thus thenumber of bits used to express lt_idx_sps[i] may be variable. Therefore,in the example shown in Table 13, the number of bits used to expresslt_idx_sps[i] may not be “ceil(log 2(num_long_term_ref_pics_sps))”.

Details of the LTRP-relevant information defined in the slice header inthe example shown in Table 13 are the same as in the example shown inTable 12 except for the above-mentioned details and thus specificdescription thereof will not be repeated.

Table 14 shows another example of the SPS including information on theSPS POC LSB.

TABLE 14 seq_parameter_set_rbsp( ) { Descriptor  ... long_term_ref_pics_present_flag u(1)  if(long_term_ref_pics_present_flag ) {   num_long_term_ref_pics_sps ue(v)  for( i = 0; i < num_long_term_ref_pics_sps; i++ ) {   lt_ref_pic_poc_lsb_sps[ i ] u(v)    used_by_curr_pic_lt_sps_flag[ i ]u(1)   }  }  ... }

In the examples shown in Tables 11 to 13, it may be determined byused_by_curr_pic_lt_flag[i] transmitted through the use of a sliceheader whether the first candidate LTRPs (for example, the LTRPs inwhich the POC LSB is determined on the basis of the SPS POC LSB out ofthe candidate LTRPs to be included in the long-term reference pictureset of the current picture) and the second candidate LTRPs (for example,the LTRPs in which the POC LSB is determined on the basis ofpoc_lsb_lt[i] explicitly transmitted through the slice header out of thecandidate LTRPs to be included in the long-term reference picture set ofthe current picture) are used as reference pictures of the currentpicture. However, since the first POC LSBs of the first candidate LTRPsmay be determined on the basis of the SPS POC LSB, it may be determinedby a particular flag transmitted through the SPS whether the firstcandidate LTRPs are used as reference pictures of the current picture.That is, flag information specifying whether the SPS POC LSB is used asa reference picture of a picture (for example, the current picture)belonging to the SPS may be transmitted through the SPS.

Referring to Table 14, used_by_curr_pic_lt_sps_flag[i] (where i is aninteger of 0 or greater) may specify whether the i-th SPS candidate LTRPis used as a reference picture of the picture using the i-th SPScandidate LTRP as the first candidate LTRP. For example, whenused_by_curr_pic_lt_sps_flag[i] equal 0, the i-th SPS candidate LTRP maynot be used as a reference picture of the picture using the i-th SPScandidate LTRP as the first candidate LTRP. At this time, the firstcandidate LTRP corresponding to the i-th SPS candidate LTRP may not beincluded in the long-term reference picture set used to construct areference picture list.

Details of the LTRP-relevant information defined in the SPS in theexample shown in Table 14 are the same as defined in the SPS of Table 11except for the above-mentioned details and thus specific descriptionthereof will not be repeated herein.

When the information on the SPS POC LSB is transmitted through the SPSas in the example shown in Table 14, the LTRP-relevant informationdefined in the slice header may be expressed, for example, as in Table15.

TABLE 15 slice_header( ) { Descriptor  ...  if(long_term_ref_pics_present_flag ) {   num_long_term_pics ue(v)   if(num_long_term_ref_pics_sps > 0 )    num_long_term_sps ue(v)  for( i = 0; i < num_long_term_sps + num_long_term_pics; i++ ) {    if( i < num_long_term_sps )     lt_idx_sps[ i ] u(v)    else {    poc_lsb_lt[ i ] u(v)     used_by_curr_pic_lt_flag[ i ] u(1)    }   delta_poc_msb_present_flag[ i ] u(1)    if(delta_poc_msb_present_flag[ i ] )     delta_poc_msb_cycle_lt[ i ] ue(v)  }  }  ... }

In the examples shown in Tables 11 to 13, used_by_curr_pic_lt_flag[i]may be applied to both the first candidate LTRPs and the secondcandidate LTRPs. That is, it may be determined byused_by_curr_pic_lt_flag[i] transmitted through the slice header whetherthe first candidate LTRPs and the second candidate LTRPs are used asreference pictures of the current picture. As described above, the firstcandidate LTRPs may be determined on the basis of lt_idx_sps[i] and thesecond candidate LTRPs may be determined on the basis of poc_lsb_lt[i].

When used_by_curr_pic_lt_sps_flag[i] is transmitted through the SPS asin the example shown in Table 14, it may be determined on the basis ofused_by_curr_pic_lt_sps_flag[i] whether the first candidate LTRPs areused as reference pictures of the current picture. For example, as theexample shown in Table 12, the POC LSB of the i-th candidate LTRP to beincluded in the long-term reference picture set of the current picture(and/or the current slice) may be lt_ref_pic_poc_lsb_sps[lt_idx_sps[i]].At this time, it may be determined on the basis of the value set asused_by_curr_pic_lt_sps_flag[lt_idx_sps[i]] whether the i-th candidateLTRP is used as a reference picture of the current picture.

Therefore, used_by_curr_pic_lt_flag[i] transmitted through the sliceheader in Table 15 may be used to determine whether the second candidateLTRPs are used as reference pictures of the current picture.Accordingly, used_by_curr_pic_lt_flag[i] may not be transmitted whenlt_idx_sps[i] is transmitted, and may be transmitted only whenpoc_lsb_lt[i] is transmitted. Referring to Table 15,used_by_curr_pic_lt_sps_flag[i] may be transmitted along withpoc_lsb_lt[i] instead of lt_idx_sps[i]. That is,used_by_curr_pic_lt_sps_flag[i] in the example shown in Table 15 mayspecify whether the second candidate LTRP determined on the basis ofpoc_lsb_lt[i] is used as a reference picture of the current picture.

Details of the LTRP-relevant information defined in the SPS in theexample shown in Table 14 are the same as defined in the SPS of Table 11except for the above-mentioned details and thus specific descriptionthereof will not be repeated herein.

Table 16 shows another example of the LTRP-relevant information definedin the slice header when the LTRP-relevant information is transmitted asin th example shown in Table 14.

TABLE 16 slice_header( ) { Descriptor  ...  if(long_term_ref_pics_present_flag ) {   num_long_term_pics ue(v)   if(num_long_term_ref_pics_sps > 0 )    num_long_term_sps ue(v)  for( i = 0; i < num_long_term_sps + num_long_term_pics; i++ ) {    if( i < num_long_term_sps )     lt_idx_sps[ i ] ue(v)    else {    poc_lsb_lt[ i ] u(v)     used_by_curr_pic_lt_flag[ i ] u(1)    }   delta_poc_msb_present_flag[ i ] u(1)    if(delta_poc_msb_present_flag[ i ] )     delta_poc_msb_cycle_lt[ i ] ue(v)  }  }  ... }

In the example shown in Table 15, u(v) is set as a descriptor oflt_idx_sps[i]. Accordingly, a “fixed length encoding” method based on afixed number of bits v may be applied to lt_idx_sps[i]. However, sincean LTRP has a feature that it is not changed in a very long period, itmay be inefficient to apply the fixed length encoding to lt_idx_sps[i].

Therefore, an encoding method of adaptively determining a bit length maybe applied to lt_idx_sps[i]. An example of the encoding method ofadaptively determining a bit length may be an exponential golombencoding method.

Referring to Table 16, ue(v) may be set as a descriptor oflt_idx_sps[i]. That is, lt_idx_sps[i] may be encoded and transmittedusing ue(v) through the use of slice header. ue(v) may specify a syntaxelement to be encoded on the basis of the exponential golomb encodingmethod. Accordingly, in the example shown in Table 16, lt_idx_sps[i] maybe encoded on the basis of an exponential golomb code and may betransmitted to the video decoder. Here, the exponential golomb code maybe, for example, a 0-th order exponential golomb code.

When the fixed length encoding method is applied as in the example shownin Table 16, the number of bits used to express lt_idx_sps[i] may be“ceil(log 2(num_long_term_ref_pics_sps))”. However, when the exponentialgolomb encoding method is used, the bit length may be variablydetermined on the basis of the exponential golomb code and thus thenumber of bits used to express lt_idx_sps[i] may be variable. Therefore,in the example shown in Table 16, the number of bits used to expresslt_idx_sps[i] may not be “ceil(log 2(num_long_term_ref_pics_sps))”.

Details of the LTRP-relevant information defined in the slice header inthe example shown in Table 16 are the same as in the example shown inTable 15 except for the above-mentioned details and thus specificdescription thereof will not be repeated.

FIG. 6 is a flowchart schematically illustrating an encoding procedurewhich is performed by a video encoder according to the presentinvention. The video encoder performing the encoding procedureillustrated in FIG. 6 may correspond to the video encoder describedabove with reference to FIG. 1.

Referring to FIG. 6, the video encoder may perform a predictionoperation on a current block (S610). The video encoder may perform interprediction or intra prediction on the current block. When the interprediction is performed, reference pictures to be used to motioncompensation of the current block may be determined on the basis of areference picture list. Here, the reference picture list may beconstructed or initialized on the basis of the reference picture set asdescribed above. The examples of the procedure of constructing areference picture set and the procedure of constructing a referencepicture list on the basis of the reference picture set are describedabove and thus description thereof will not be repeated herein.

The video encoder may transform/quantize the prediction result on thecurrent block (S620). At this time, the video encoder maytransform/quantize a residual block corresponding to a differencebetween the prediction result and the original block.

The video encoder may entropy-encode the transformed/quantizedinformation (S630). At this time, the video encoder may entropy-encodeinformation for constructing the reference picture set together. CABACmay be used as the entropy encoding method.

The video encoder may signal the entropy-encoded information (S640). Atthis time, the signaled information may include information forconstructing the reference picture set for the current picture (and/orthe current block). The reference picture set may be constructed foreach slice (and/or picture) and information for constructing thereference picture set may be transmitted in a state where it is includedin the SPS, the PPS, and/or the slice header as described above. Thespecific examples of the information for constructing the referencepicture set are described above and specific description thereof willnot be repeated herein.

While the operations of the video encoder are schematically describedwith reference to FIG. 6 so as to easily understand the presentinvention in consideration of the details of the reference picture setand the reference picture list, this is for convenience of explanationand the operations of the video encoder according to the presentinvention may include the operations described with reference to FIG. 1.

FIG. 7 is a flowchart schematically illustrating a decoding procedurewhich is performed by a video decoder according to the presentinvention. The video decoder performing the decoding procedureillustrated in FIG. 7 may correspond to the video decoder describedabove with reference to FIG. 2.

Referring to FIG. 7, the video decoder may receive a bit stream from thevideo encoder and may perform an entropy decoding operation (S710). Thebit stream received from the video encoder may include information forconstructing a reference picture set. As described above, theinformation for constructing the reference picture set may be receivedin a state where it is included in the SPS, the PPS, and/or the sliceheader. The specific examples of the information for constructing thereference picture set are described above and thus specific descriptionthereof will not be repeated herein.

The video decoder may perform a prediction operation on the currentblock on the basis of the entropy-decoded information (S720). Theprediction method for the current block may be transmitted from thevideo encoder. When the prediction method for the current block is interprediction, the video decoder may perform the prediction operation usingthe reference picture list constructed on the basis of the receivedinformation.

Here, the reference picture list may be constructed or initialized onthe basis of the reference picture set as described above. The examplesof the procedure of constructing a reference picture set and theprocedure of constructing a reference picture list on the basis of thereference picture set are described above and thus description thereofwill not be repeated herein. The constructed reference picture list maybe stored in a memory of the video decoder.

The video decoder may reconstruct a video (S730). The video decoder mayreconstruct the current block on the basis of the prediction on thecurrent block and may reconstruct a video using the reconstructedblocks. When a skip mode is used, the residual signal is not transmittedand thus the predicted block may be used as a reconstructed block. Whena merge mode or an MVP mode is used, the video decoder may reconstructthe current block by adding the predicted block and the residual block.

In this description, terms such as a “picture included in a referencepicture set”, an “x-th picture in a reference picture set”, a “pictureincluded in a reference picture list”, and an “x-th picture in areference picture list” are used, but these terms are intended only forconvenience of explanation. A picture in the reference picture set or areference picture list may be a picture of which corresponding POCinformation is included in the reference picture set or the referencepicture list. The x-th picture in the reference picture set or the x-thpicture in the reference picture list may be a picture in which thecorresponding POC information is arranged in the reference picture setor the reference picture list.

While the methods in the above-mentioned embodiments have been describedon the basis of the flowcharts as a series of steps or blocks, theinvention is not limited to the order of the steps and a certain stepmay be performed in an order other than described above or at the sametime as described above. The above-mentioned embodiments include variousexamples. Therefore, the invention includes all substitutions,corrections, and modifications belonging to the appended claims.

When it is mentioned above that an element is “connected to” or “coupledto” another element, it should be understood that still another elementmay be interposed therebetween, as well as that the element may beconnected or coupled directly to another element. On the contraiy, whenit is mentioned that an element is “connected directly to” or “coupleddirectly to” another element, it should be understood that still anotherelement is not interposed therebetween.

1. A video decoding method comprising the steps of: acquiringinformation for constructing a reference picture set of a currentpicture by entropy-decoding received bitstream information; andperforming a prediction on a prediction target block in the currentpicture using a reference picture list constructed on the basis of thereference picture set, wherein the information for constructing thereference picture set includes first flag information specifying whethera long-term reference picture (LTRP) is used to perform an interprediction on at least one picture belonging to a current sequence, andwherein the first flag information is transmitted through a sequenceparameter set (SPS) sequence by sequence.
 2. The video decoding methodaccording to claim 1, wherein the first flag information specifies thata long-term reference picture (LTRP) is used to perform an interprediction on at least one picture belonging to the current sequence,wherein the reference picture set includes a long-term reference picture(LTRP) set, and wherein the information for constructing the referencepicture set further includes information for deriving a picture ordercount (POC) of an LTRP to be included in the LTRP set.
 3. The videodecoding method according to claim 2, wherein the information forderiving the POC includes a sequence parameter set (SPS) LTRP indexinformation specifying an LTRP to be included in the LTRP set among atleast one of SPS candidate LTRPs to be transmitted through an SPS,wherein the POC of the LTRP to be included in the LTRP set is derived onthe basis of a least significant bit (LSB) of the POC of the LTRPspecified by the SPS LTRP index information, and wherein the SPS LTRPindex information is transmitted through a slice header corresponding tothe current picture.
 4. The video decoding method according to claim 3,wherein the information for constructing the reference picture setincludes second flag information specifying whether the LTRP specifiedby the SPS LTRP index information is used as a reference picture of apicture belonging to the current sequence, wherein the LTRP set does notinclude the LTRP specified by the SPS LTRP index information when thesecond flag information specifies that the LTRP specified by the SPSLTRP index information is not used as a reference picture of the picturebelonging to the current sequence, and wherein the second flaginformation is transmitted through a SPS sequence by sequence.
 5. Thevideo decoding method according to claim 3, wherein the SPS LTRP indexinformation is encoded and transmitted on the basis of a fixed number ofbits.
 6. The video decoding method according to claim 2, wherein theinformation for deriving the POC includes PCO LSB information specifyingthe LSB of the POC of the LTRP to be included in the LTRP set, whereinthe POC of the LTRP to be included in the LTRP set is derived on thebasis of the POC LSB information, and wherein the POC LSB information istransmitted through a slice header corresponding to the current picture.7. The video decoding method according to claim 6, wherein theinformation for constructing the reference picture set includes thirdflag information specifying whether the LTRP having the POC derived onthe basis of the POC LSB information is used as a reference picture ofthe current picture, wherein when the third flag information specifiesthat the LTRP having the POC derived on the basis of the POC LSBinformation is not used as a reference picture of the current picture,the LTRP set does not include the LTRP having the POC derived on thebasis of the POC LSB information, and wherein the third flag informationis transmitted through a slice header corresponding to the currentpicture.
 8. A video decoder comprising: an entropy decoding module thatacquires information for constructing a reference picture set of acurrent picture by entropy-decoding received bitstream information; aprediction module that constructing a predicted block corresponding to aprediction target block in the current picture by performing aprediction on the prediction target block using a reference picture listconstructed on the basis of the reference picture set; and areconstructed block constructing unit that constructs a reconstructedblock on the basis of the predicted block, wherein the information forconstructing the reference picture set includes first flag informationspecifying whether a long-term reference picture (LTRP) is used toperform an inter prediction operation on at least one picture belongingto a current sequence, and wherein the first flag information istransmitted through a sequence parameter set (SPS) sequence by sequence.9. A video information transmitting method comprising the steps of:performing an inter prediction on a current picture using a referencepicture list; and entropy-encoding and transmitting information forconstructing a reference picture set used to construct the referencepicture list, wherein the information for constructing the referencepicture set includes first flag information specifying whether along-term reference picture (LTRP) is used to perform an interprediction operation on at least one picture belonging to a currentsequence, and wherein the step of entropy-encoding and transmitting theinformation includes transmitting the first flag information through asequence parameter set (SPS) sequence by sequence.
 10. The videoinformation transmitting method according to claim 9, wherein the firstflag information specifies that a long-term reference picture (LTRP) isused to perform an inter prediction operation on at least one picturebelonging to the current sequence, wherein the reference picture setincludes a long-term reference picture (LTRP) set, and wherein theinformation for constructing the reference picture set further includesinformation for deriving a picture order count (POC) of an LTRP to beincluded in the LTRP set.
 11. The video information transmitting methodaccording to claim 10, wherein the information for deriving the POCincludes a sequence parameter set (SPS) LTRP index informationspecifying an LTRP to be included in the LTRP set among at least one ofSPS candidate LTRPs to be transmitted through an SPS, and wherein theentropy-encoding and transmitting information for constructing areference picture set includes transmitting SPS POC least significantbit (LSB) information specifying an LSB of the POC of each of SPScandidate LTRPs through the SPS sequence by sequence and transmittingthe SPS LTRP index information through a slice header corresponding tothe current picture.
 12. The video information transmitting methodaccording to claim 11, wherein the information for constructing thereference picture set includes second flag information specifyingwhether the LTRP specified by the SPS LTRP index information is used asa reference picture of a picture belonging to the current sequence,wherein the LTRP set does not include the LTRP specified by the SPS LTRPindex information when the second flag information specifies that theLTRP specified by the SPS LTRP index information is not used as areference picture of the picture belonging to the current sequence, andwherein the entropy-encoding and transmitting information forconstructing a reference picture set includes transmitting the secondflag information through a SPS sequence by sequence.
 13. The videoinformation transmitting method according to claim 11, wherein theentropy-encoding and transmitting information for constructing areference picture set includes entropy-encoding the SPS LTRP indexinformation on the basis of a fixed number of bits.
 14. The videoinformation transmitting method according to claim 10, wherein theinformation for deriving the POC includes PCO LSB information specifyingthe LSB of the POC of the LTRP to be included in the LTRP set, andwherein the entropy-encoding and transmitting information forconstructing a reference picture set includes transmitting the POC LSBinformation through a slice header corresponding to the current picture.15. The video information transmitting method according to claim 14,wherein the information for constructing the reference picture setincludes third flag information specifying whether the LTRP having thePOC corresponding to the POC LSB information is used as a referencepicture of the current picture, wherein when the third flag informationindicates that the LTRP having the POC corresponding to the POC LSBinformation is not used as a reference picture of the current picture,the LTRP set does not include the LTRP having the POC derived on thebasis of the POC LSB information, and wherein the entropy-encoding andtransmitting information for constructing a reference picture setincludes transmitting the third flag information through a slice headercorresponding to the current picture.
 16. A video informationtransmitting device comprising: a prediction module that performs aninter prediction on a current picture using a reference picture list;and an entropy encoding module that entropy-encodes and transmitsinformation for constructing a reference picture set used to constructthe reference picture list, wherein the information for constructing thereference picture set includes first flag information specifying whethera long-term reference picture (LTRP) is used to perform an interprediction on at least one picture belonging to a current sequence, andwherein the entropy encoding module transmits the first flag informationthrough a sequence parameter set (SPS) sequence by sequence.